In fact, I was working on my own Modern Masters set (as a fun exercise in MtG Forge), and I had exactly this rarity shift in mind:
These rarity downshifts are magnificent! I'm weeping tears of joy here! Let me play with these please!
That depends a bit on your playstyle I think, but this is generally true for each shard/wedge when you look beyond the first one or two most interecting cards. I heavily improvised for my 3-color themed cube and designed a good bit of 3-color cards myself.
The whole spoiler is up: http://magic.wizards.com/en/articles/archive/card-image-gallery/modernmasters2015edition
This looks like an absolute blast to draft. Too bad there's a few really expensive rares and mythic that drive up the booster price to ridiculous heights.
Black ZOMBIES! Now all your tribal lords will work. Oh wait... We replaced [tribe] with [creature]. Never mind. Creature Grenade is still a great name though!
Also, even when it works, her effect isn't really fun at all. 'Oh great, turn 4/5 Griselbrand? Why not, look at me having fun!" She's ruined plenty of commander games around the globe, I'm not about to introduce her to my cube.
Dralnu's Crusade
Deadapult
Lord of the Undead
Goblin Grenade
Creature Grenade {R}
Sorcery
As an additional cost to cast Creature Grenade, sacrifice a creature.
Creature Grenade deals 5 damage to target creature or player.
Don't underestimate the aerodynamic qualities of the common creature.
I am confident they will eventually go there. Wasn't there a setting during the Great Designer Search... [searching]
Yep. Ethan Fleischer had a block that started in the prehistoric age. He's still with WotC, and one of his mechanics (evolve) already made it to print. I bet one of these days...