I mean, it's super aggressive. I can't think of any spell that does 5 damage for 3 mana without an additional cost. Given Eradicate it'd probably be okay (if still super aggressive) at 2BR though.
Yeah, after I posted it, I realized that 2BR may be better, since 5 damage does destroy a ton of relevant creatures this Standard and it would be a bit much to have a Lobotomize effect added onto that at 3 mana.
Anyway, last guesses for new mechanics?
not!Jeskai / Ojutai - It should be something that connects to prowess, since that is still in Fate Reforged, meaning it's likely something that relates to noncreatures. However, my thoughts went to the recent Uncharted Realms story, with alternate Taigam pushing away Sarkhan with a force punch. It could be something similar to cycling or even reinforce, which have the goal of allowing you to include cards that may not be useful in all cases. I also expect it to be pretty boring, so I came up with a card like this:
Monk Powers - 3W
Enchantment - Aura
Enchant creature.
Enchanted creature gets +2/+2 and vigilance.
Mindpunch W
(W, Discard this card: Tap target creature.)
not!Sultai / Silumgar - Since it's been mentioned that Sultai Emissary plays well with this mechanic, and it probably isn't just morbid, perhaps it relates to exiling a creature card from your graveyard as a kicker cost.
Zombie Guard - 1B
Creature - Zombie
Gravekicker
(You may exile a creature card from your graveyard as you cast this spell)
If Zombie Guard was gravekicked, it enters the battlefield with a +1/+1 counter on it.
1/2
not!Temur / Atarka - The flavor of Temur now bringing offerings to Atarka to appease her does seem to imply a similar effect to the above, since they probably won't bring back offering or devour. Maybe instead of a "kicker" like devour, it can be an alternate cost, like a less restricted offering.
Hungry Rhino - 3GG
Creature - Rhino
Snare 2GG
(As an alternate cost, you may pay 2GG and sacrifice a creature. If you do, you may cast this card at any time you could cast an instant.)
Trample
When Hungry Rhino enters the battlefield, if you paid the snare cost, you gain 3 life.
5/5