Definitely various possibilities with legendary matters in general.
Cultist of Aoskar GG
Creature - Human Priest
When Cultist of Aoskar enters the battlefield, you may search your library for a legendary land card and put it into your hand, then shuffle your library. T: Add one mana of any color to your mana pool. You may only spend this mana on legendary spells.
1/1
Maybe have the equivalent of devotion for legendaries, which is the opposite of the legend-as-singular-dude theme? Like effects based on X, where X = the number of legendary permanents you control? Or would that be too redundant given the nature of the set?
You could also use a great big non-legendary board wiper and/or vice versa.
Gauntlet of Heroes 4WW
Sorcery
Destroy all non-legendary permanents.
W is Lawful U is Neutral B is Evil R is Chaotic G is Good
This may not be especially relevant here (or anywhere), but it's fun food for thought.
Maybe my scheme wasn't as good as yours, but I brought up something similar in one of my first threads here, related to the idea of a Planescape based MTG set. There were some strong reasons given for shooting down the attempt to re-do the color pie based on the alignment system. While white and black map to good and evil pretty well, and red is chaos, I wanted blue to be true lawful, but white already has it. I wanted green to be true neutrality (nature's neutrality) but it got convoluted. There were too many incongruities for it to be a clean thing. Especially when I tried to represent the planes. Mechanus does not feel mono-white! In the case of your scheme, think mono-green being good is a stretch.
I still think there are possibilities in taking stuff from D&D though.
Change "when you cast Venser" to "When Venser enters the battlefield". Not entirely sure how I feel about it having flash, even considering that it's color shifted. Otherwise it feels decent.
Doom Meteor2BR
Sorcery {R}
Each opponent must sacrifice a land and loses life equal to the total of lands sacrificed.
Perhaps Replicant Shaman would be more fair at 1G or GG? I originally just had it as a Changeling creature that had a Reflecting Pool effect, which is more balanced for one mana. But compared to Birds of Paradise and similar one mana creatures that tap for any color it seemed weak. Then I added on the other part to make it more versatile and powerful, which probably drove it over the top.
I like Peresa, Forest Caller. Not sure if it's too expensive, though anything that taps for mana at four mana feels a little late as far as the mana effect. The costing 1 less part probably makes it fair at that cost, but I could see it flying at 3 mana if you make it 2/2. Interesting that it's based on legendary and planeswalkers. Looks perfect for epic creatures and super-friends.
Eldritch Summoner2G
Creature - Human Shaman {R}
When Eldritch Summoner enters the battlefield, you may search your library for a creature card with converted mana cost 3 or less and put it into your hand, then shuffle your library. T: Add one mana of any color to your mana pool. You may only use this mana to cast creatures with converted mana cost 3 or less.
1/1
I can see Dark Acolyte possibly flying, though maybe uncommon would be better? Then again, that life loss can add up if you're using multiples of these. It's not like you're bringing back Dark Ritual, from the days when the color pie was more of an experimental color stew.
The one other way I had thought of in a few previous card designs for black, in order to justifiably get mana accelteration, was of the following form:
"T: Add B to your mana pool. You may only use this mana to cast spells from graveyards".
Definitely like the concept behind Pleasure In Pain. As others have noted, it is pretty abusable though. Maybe I'd be inclined do "at the beginning of *each* end step, if you lost life this turn, you may draw a card". Which is Piar's version above. This both limits it to one card draw per turn and makes it applicable on your opponent's turns still, while not allowing you to continually ping yourself for cards on your own turn. Just some ideas.
A more versatile Rebuff The Wicked. While obviously white counterspells are not normal, there is some precedent for it and I suppose it fits the flavor if we're going in that direction. Simple enough and makes sense for white.
Replicant ShamanG
Creature - Shapeshifter (R)
Changeling
When Replicant Shaman enters the battlefield, choose a basic land type. Lands you control are that type in addition to their other types. T: Add to your mana pool one mana of any type that a land you control could produce.
1/1
I like Hex a lot. It's similar to but a more refined variation on what I was trying to do with Infiltrate when I posted this card:
Arcane InfiltratorB
Creature - Human Rogue
Infiltrate 1BB (You may cast this creature for its Infiltrate cost. If you do, it enters the battlefield under the control of target opponent.)
Arcane Infiltrator is unblockable.
Spells you cast cost an additional 1 life.
2/2
One worry is that this kind of mechanic would be hard pressed to be put on commons or too many cards, as Viridian also mentions.
I like the card and overall idea. Intuitively though, it makes more sense to me as an aura rather than an instant that has to remind you of a non-instant effect. Though if it were an aura it would have to also have flash in order to work.
With card draw this could be a little overpowered I think. Maybe not a turn 2-3 win but one or two of these placed right means a player is dead. Otherwise its a cheap and quick way to kill off some creatures on top of that. I might be more interested in a somewhat more expensive casting cost (maybe you have to pay X mana too?) and making it give -1/-1 counters.
Jelinar, Arena Master 2GR
Legendary Creature - Human Warrior {R}
Whenever a creature enters the battlefield under your control, you may have it fight target creature you don't control. Whenever a non-token creature you control wins a fight against another creature, put a +1/+1 counter on that creature.
4/4
So a 5/5 flyer for 4 mana with Dash that also returns some attacking creatures to your hand if they're hasted or have Dash, in exchange for other creatures from your hand hasted for free. It effectively seems to give any hasted creature the equivolent of Dash (return to hand at end of turn). Interesting.
Spellthief 2UR
Creature - Human Rogue {R}
Flash
When Spellthief enters the battlefield, counter target instant or sorcery spell and exile it encoded onto Spellthief. Whenever Spellthief deals combat damage to a player, you may cast a copy of the encoded spell without paying its mana cost.
2/2
Interesting card. I like the overall effect, not sure about five cards or not. But Gate seems too limited to be a keyword, without anything else going on. That is, once you have one card with Gate there's no purpose to having another card with gate, unless there's some other effect related to the top card of your library. Gate itself is redundant as a keyword.
Scepter of Urgosh 2RR
Legendary Artifact {M}
Whenever a land enters the battlefield under an opponent's control, Scepter of Urgosh deals 2 damage to that player. T: Scepter of Urgosh deals 2 damage to target player. You may only use this ability if that player lost a land this turn.
Effectively, this is Ankh of Mishra just for your opponents with a cool bonus. Too much hate in one card?
Duality is the one of most interest to me. But I think as worded Duality doesn't work for permanents. Directly coping spells generally applies to instants and sorceries. To make it applicable to permanents, in my understanding you'd have to re-word it to also produce token copies in the case of permanents. I'm also not sure how applicable that is outside of creatures, at least in pre-existing magic precedent (are there cards that produce token copies of non-creature permanents?).
Cultist of Aoskar GG
Creature - Human Priest
When Cultist of Aoskar enters the battlefield, you may search your library for a legendary land card and put it into your hand, then shuffle your library.
T: Add one mana of any color to your mana pool. You may only spend this mana on legendary spells.
1/1
Maybe have the equivalent of devotion for legendaries, which is the opposite of the legend-as-singular-dude theme? Like effects based on X, where X = the number of legendary permanents you control? Or would that be too redundant given the nature of the set?
You could also use a great big non-legendary board wiper and/or vice versa.
Gauntlet of Heroes 4WW
Sorcery
Destroy all non-legendary permanents.
Maybe my scheme wasn't as good as yours, but I brought up something similar in one of my first threads here, related to the idea of a Planescape based MTG set. There were some strong reasons given for shooting down the attempt to re-do the color pie based on the alignment system. While white and black map to good and evil pretty well, and red is chaos, I wanted blue to be true lawful, but white already has it. I wanted green to be true neutrality (nature's neutrality) but it got convoluted. There were too many incongruities for it to be a clean thing. Especially when I tried to represent the planes. Mechanus does not feel mono-white! In the case of your scheme, think mono-green being good is a stretch.
I still think there are possibilities in taking stuff from D&D though.
Doom Meteor 2BR
Sorcery {R}
Each opponent must sacrifice a land and loses life equal to the total of lands sacrificed.
Simple one.
I like Peresa, Forest Caller. Not sure if it's too expensive, though anything that taps for mana at four mana feels a little late as far as the mana effect. The costing 1 less part probably makes it fair at that cost, but I could see it flying at 3 mana if you make it 2/2. Interesting that it's based on legendary and planeswalkers. Looks perfect for epic creatures and super-friends.
Eldritch Summoner 2G
Creature - Human Shaman {R}
When Eldritch Summoner enters the battlefield, you may search your library for a creature card with converted mana cost 3 or less and put it into your hand, then shuffle your library.
T: Add one mana of any color to your mana pool. You may only use this mana to cast creatures with converted mana cost 3 or less.
1/1
Kind of the weenie tutor alternative to Peresa.
The one other way I had thought of in a few previous card designs for black, in order to justifiably get mana accelteration, was of the following form:
"T: Add B to your mana pool. You may only use this mana to cast spells from graveyards".
This gives it a flavorful in-color limitation.
Replicant Shaman G
Creature - Shapeshifter (R)
Changeling
When Replicant Shaman enters the battlefield, choose a basic land type. Lands you control are that type in addition to their other types.
T: Add to your mana pool one mana of any type that a land you control could produce.
1/1
Arcane Infiltrator B
Creature - Human Rogue
Infiltrate 1BB (You may cast this creature for its Infiltrate cost. If you do, it enters the battlefield under the control of target opponent.)
Arcane Infiltrator is unblockable.
Spells you cast cost an additional 1 life.
2/2
One worry is that this kind of mechanic would be hard pressed to be put on commons or too many cards, as Viridian also mentions.
Jelinar, Arena Master 2GR
Legendary Creature - Human Warrior {R}
Whenever a creature enters the battlefield under your control, you may have it fight target creature you don't control. Whenever a non-token creature you control wins a fight against another creature, put a +1/+1 counter on that creature.
4/4
Spellthief 2UR
Creature - Human Rogue {R}
Flash
When Spellthief enters the battlefield, counter target instant or sorcery spell and exile it encoded onto Spellthief. Whenever Spellthief deals combat damage to a player, you may cast a copy of the encoded spell without paying its mana cost.
2/2
Scepter of Urgosh 2RR
Legendary Artifact {M}
Whenever a land enters the battlefield under an opponent's control, Scepter of Urgosh deals 2 damage to that player.
T: Scepter of Urgosh deals 2 damage to target player. You may only use this ability if that player lost a land this turn.
Effectively, this is Ankh of Mishra just for your opponents with a cool bonus. Too much hate in one card?