Sentinel X: Enters the battlefield with X slumber counters on it. Whenever you take damage from a source owned by another player, you may remove 1 slumber counter from this creature. For as long as there is a slumber counter on this creature, it has defender.
I really don't see this being versatile enough in order for it to be a new set mechanic. It's quite controlling of the game around itself.
It's also a downside mechanic, which are looked upon very unfavourably.
I would prefer to see using defender as a cost for some reward like +1/+1 counters or an open ended version with life gain and card draw and such.
Decay X: Put X decay counters on this creature. At the beginning of your upkeep, remove a decay counter from this creature. Then, when there are no more decay counters on this creature, sacrifice it.
This is pretty much just Vanishing.
On your returning mechanics
Deathtouch, Hexproof, Vigilance, Flying and Double Strike cannot be returning mechanics. They are evergreen mechanics. They can and should be used in whatever set.
Untap (untap abilities)
This is not a named mechanic.
A questionable choice for a return. Annihilator is not a very fun mechanic on balance.
Phyrexian Mana (on 1 card)
Returning a set mechanic for use on 1 card is not a good idea. Phyrexian mana can easily be impersonated on an individual card basis. If you want to use Phyrexian mana it should be on something like 10 or 15 cards.
Phyrexian mana is also an explicit reference to Phyrexia. It should not be used unless Phyrexia is a large part of the set.