MTG Salvation

Mechanics
-- Dominance - As long as you have more life than each opponent, [effect].
-- Exhume (You may cast this spell from your graveyard by sacrificing a creature in addition to its other costs.)
-- Reverberate {cost} (As you cast an instant or sorcery spell, you may reveal this card from your hand and pay its Reverberate cost. If you do, add this card's effects to that spell.)
-- Envision — {cost}, Reveal [cardname] from your hand: [effect]. (Activate this ability only once each turn.)
-- Warcast (This spell cost 1 less to cast for each creature that attacked this turn.)

I really like the set great start. I noticed you haven't listed all of the mechanics like emerge. UG Emerge: play card as an instant if you return a permanent you control (is this supposed to be a non-land permanent that is returned?, it might be too broken if you could always activate it with lands)

As for the mechanics dominance is pretty great and a good fit for BW.
Reverberate is also quite good and shouldn't be too bad as long as mana production in UR doesn't become too easy.
As for envision what color combination is it, i can't seem to find an example for it. Seems WG as it is the only clan color combo left, but i don't think it fits well with the GW flavor. I assume its envision cost is actually somewhat higher than its normal cost but not quite convoke high. Also is this effect sorcery speed only? because once a turn is basically instant speed, does it mean you are limiting copying effect interactions? Would this be better as an overarching mechanic for the set since each clan envisions a future led by them. Seems like a great small-medium mechanic stretching all the colors. Maybe fixing its cost or effect to a standard value will give the mechanic an identity. For example adding a +1/+1 counter to target creature if you cast a card for its envision cost, or tapping X creature(s) as a standard cost not to make it cheaper but just to allow you to reuse it more.
Warcast seems great but the most overpowered as well. The effects listed are too good and like others pointed out early creature aggro is pretty easy to do with cheap creatures. I think increasing the mana costs or nerfing the spell effects needs to happen to balance its power level.

Also don't forget to make base stats of creatures match with color identity, green should have the best raw stats overall and red and white shouldn't be better at any time except perhaps early game.