I think that based on the comments so far, my instinct is to:
1) Add a "you may" to the exiling cards from your library.
2) Not do too many cards with this at low rarities, or in total for that matter
3) Find either a graveyard mechanic or a library-manipulation/peeking mechanic to pair with it.
So here's the mechanic:
EDIT: underlined was added after comments
Sift #(At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top # card(s) of your library.)
Then the cards have abilities based on the card(s) exiled with them.
There are a bunch of cards I just brainstormed in the spoiler below. My main questions are these:
1) Is it even interesting?
2) Would it be better if the cards were put on the bottom of the library?
3) Is it broken? Would the change in the above question make it more or less broken?
4) Is this all irrelevant because it's too wordy/complicated?
Gilded-Mind Cavalry3W
Creature — Human Knight (U)
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.)
Gilded-Mind Cavalry has vigilance and lifelink as long as an enchantment card is exiled with it.
3/4
Noble Herald1W
Creature — Human Cleric (R)
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Permanents you control with converted mana cost less than the highest converted mana cost among cards exiled with Noble Herald have indestructible.
1/4
Fora, Meddlesome MageUU
Legendary Creature — Human Wizard (R)
Hexproof
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Your opponents can’t cast spells that share a type with a card exiled with Fora, Meddlesome Mage.
2/2
Spellwaft Scout2U
Creature — Human Scout (C)
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Spellwaft Scout has flying as long as an instant or sorcery card is exiled with it.
2/3
Noxious Walker1B
Creature — Zombie (C)
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.)
Whenever Noxious Walker becomes blocked, defending player loses 2 life for each Swamp card exiled with it.
3/1
Ruinous Specter2BB
Creature — Specter (R)
Flying
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Whenever Ruinous Specter deals combat damage to a player, that player sacrifices a permanent that shares a type with a card exiled with Ruinous Specter.
2/2
Fury Seeker3R
Creature — Human Warrior (C)
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Fury Seeker has first strike as long as a creature card is exiled with it.
4/2
Hellkite of the Peaks3RR
Creature — Dragon (R)
Flying
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.)
Whenever Hellkite of the Peaks attacks, it deals 4 damage to target creature or player for each Mountain card exiled with it.
4/4
Determined Trailblazer2G
Creature — Human Scout (C)
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.)
Determined Trailblazer gets +1/+1 for each land card exiled with it.
2/2
Moody Baloth3G
Creature — Beast (R)
Trample
Sift 3 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top three cards of your library.)
Moody Baloth gets +X/+0, where X is the total converted mana cost of cards exiled with it.
0/5
Metal-Shard Hourglass3
Artifact (R)
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.) T: Look at the top X cards of your library, where X is the highest converted mana cost among artifact cards exiled with Metal-Shard Hourglass. Put one into your hand and the rest on the bottom of your library in random order.
Any specific criticisms on the cards in the above spoiler are also appreciated.
Interplanar Edict1B
Enchantment (R)
Whenever an opponent rolls ,, that player discards a card and sacrifices a permanent. B, Sacrifice Interplanar Edict: Target player sacrifices a creature.
You Are What You Eat1GU
Instant (R)
Until end of turn, whenever a creature dies, each creature on the battlefield that dealt combat damage to it this turn becomes a copy of it. (This effect lasts indefinitely)
Meddling InquisitorUB
Creature - Human Wizard (R)
Hexproof
When Meddling Inquisitor enters the battlefield, target opponent chooses a nonland card name. Until your next turn, that player can’t cast spells without the chosen name.
2/2
Stern Administrator1W
Creature - Human Advisor (R)
Each player cannot activate abilities of cards if the total mana cost of that activation is greater than the converted mana cost of the card.
2/2
Bloody Recruitment3B
Instant (R)
Create X 1/1 black Vampire creature tokens with lifelink, where X is half the life you’ve lost this turn, rounded down.
Pompous Skywatch1WU
Creature - Human Soldier (U) Will of the council — Whenever Cloudwatch Comrades attacks, starting with you, each player votes for high or mighty. If high gets more votes, attacking creatures you control gain flying until end of turn. If mighty gets more votes or the vote is tied, attacking creatures you control get +1/+1 until end of turn.
2/3
Zealous Executioner2B
Creature - Human Mercenary (U)
When Zealous Executioner enters the battlefield, if it isn't a token, each player sacrifices a creature.
Embalm 1W
2/2
Crippling ClericRW
Creature - Human Shaman (R)
Whenever a creature is dealt noncombat damage, that creature can’t attack or block this turn. 3, T: Crippling Cleric deals 1 damage to target creature and you gain 1 life.
2/2
Fastidious Surveyor1GG
Creature - Human Druid (U)
Fastidious Surveyor has indestructible as long as you control more lands that share a name than each opponent.
3/2
Genesis Pulse2GW
Sorcery (R)
Each player chooses an artifact, a creature, an enchantment, and a land from among the permanents he or she controls, then creates a token that’s a copy of each.
1) Add a "you may" to the exiling cards from your library.
2) Not do too many cards with this at low rarities, or in total for that matter
3) Find either a graveyard mechanic or a library-manipulation/peeking mechanic to pair with it.
EDIT: underlined was added after comments
Sift # (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top # card(s) of your library.)
Then the cards have abilities based on the card(s) exiled with them.
There are a bunch of cards I just brainstormed in the spoiler below. My main questions are these:
1) Is it even interesting?
2) Would it be better if the cards were put on the bottom of the library?
3) Is it broken? Would the change in the above question make it more or less broken?
4) Is this all irrelevant because it's too wordy/complicated?
Gilded-Mind Cavalry 3W
Creature — Human Knight (U)
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.)
Gilded-Mind Cavalry has vigilance and lifelink as long as an enchantment card is exiled with it.
3/4
Noble Herald 1W
Creature — Human Cleric (R)
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Permanents you control with converted mana cost less than the highest converted mana cost among cards exiled with Noble Herald have indestructible.
1/4
Fora, Meddlesome Mage UU
Legendary Creature — Human Wizard (R)
Hexproof
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Your opponents can’t cast spells that share a type with a card exiled with Fora, Meddlesome Mage.
2/2
Spellwaft Scout 2U
Creature — Human Scout (C)
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Spellwaft Scout has flying as long as an instant or sorcery card is exiled with it.
2/3
Noxious Walker 1B
Creature — Zombie (C)
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.)
Whenever Noxious Walker becomes blocked, defending player loses 2 life for each Swamp card exiled with it.
3/1
Ruinous Specter 2BB
Creature — Specter (R)
Flying
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Whenever Ruinous Specter deals combat damage to a player, that player sacrifices a permanent that shares a type with a card exiled with Ruinous Specter.
2/2
Fury Seeker 3R
Creature — Human Warrior (C)
Sift 1 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top card of your library.)
Fury Seeker has first strike as long as a creature card is exiled with it.
4/2
Hellkite of the Peaks 3RR
Creature — Dragon (R)
Flying
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.)
Whenever Hellkite of the Peaks attacks, it deals 4 damage to target creature or player for each Mountain card exiled with it.
4/4
Determined Trailblazer 2G
Creature — Human Scout (C)
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.)
Determined Trailblazer gets +1/+1 for each land card exiled with it.
2/2
Moody Baloth 3G
Creature — Beast (R)
Trample
Sift 3 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top three cards of your library.)
Moody Baloth gets +X/+0, where X is the total converted mana cost of cards exiled with it.
0/5
Metal-Shard Hourglass 3
Artifact (R)
Sift 2 (At the beginning of your upkeep, put all cards exiled with this permanent into their owners’ graveyards, then you may exile the top two cards of your library.)
T: Look at the top X cards of your library, where X is the highest converted mana cost among artifact cards exiled with Metal-Shard Hourglass. Put one into your hand and the rest on the bottom of your library in random order.
Cut Off U
Instant (U)
Counter target spell if it was the second spell cast this turn.
Foolish Gambler 1R
Creature - Goblin (R)
R, T: Exile the top two cards of your library. If they share a card type, draw a card.
1/3
Enchantment (R)
Whenever an opponent rolls ,, that player discards a card and sacrifices a permanent.
B, Sacrifice Interplanar Edict: Target player sacrifices a creature.
You Are What You Eat 1GU
Instant (R)
Until end of turn, whenever a creature dies, each creature on the battlefield that dealt combat damage to it this turn becomes a copy of it. (This effect lasts indefinitely)
Meddling Inquisitor UB
Creature - Human Wizard (R)
Hexproof
When Meddling Inquisitor enters the battlefield, target opponent chooses a nonland card name. Until your next turn, that player can’t cast spells without the chosen name.
2/2
Stern Administrator 1W
Creature - Human Advisor (R)
Each player cannot activate abilities of cards if the total mana cost of that activation is greater than the converted mana cost of the card.
2/2
Bloody Recruitment 3B
Instant (R)
Create X 1/1 black Vampire creature tokens with lifelink, where X is half the life you’ve lost this turn, rounded down.
Creature - Human Soldier (U)
Will of the council — Whenever Cloudwatch Comrades attacks, starting with you, each player votes for high or mighty. If high gets more votes, attacking creatures you control gain flying until end of turn. If mighty gets more votes or the vote is tied, attacking creatures you control get +1/+1 until end of turn.
2/3
Enchantment (R)
Whenever a player attacks with three or more creatures, you create a tapped 2/2 black Zombie creature token.
IIW: the word "ability" or "abilities" is in the rules text.
Zealous Executioner 2B
Creature - Human Mercenary (U)
When Zealous Executioner enters the battlefield, if it isn't a token, each player sacrifices a creature.
Embalm 1W
2/2
Crippling Cleric RW
Creature - Human Shaman (R)
Whenever a creature is dealt noncombat damage, that creature can’t attack or block this turn.
3, T: Crippling Cleric deals 1 damage to target creature and you gain 1 life.
2/2
Fastidious Surveyor 1GG
Creature - Human Druid (U)
Fastidious Surveyor has indestructible as long as you control more lands that share a name than each opponent.
3/2
Genesis Pulse 2GW
Sorcery (R)
Each player chooses an artifact, a creature, an enchantment, and a land from among the permanents he or she controls, then creates a token that’s a copy of each.