Crimson Vow features a large number of mechanics. There's cleave, Blood tokens, and double-faced cards, plus faction mechanics for each ally-color tribe that doesn't have one yet: disturb for UW Spirits, exploit for UB Zombies, and training for GW Humans. Interestingly they're all adaptations or evolutions of existing mechanics: training is the inverse of mentor, disturb is the Midnight Hunt mechanic but with Auras on the back side, and exploit is reused from Tarkir's Silumgar faction without any further update.
Imagine you've been tasked with finding a mechanic to reuse or adapt for an enemy-color tribe in a future set. Figure out what that mechanic is and how (or whether) you'll tweak it, then design a mono-colored creature belonging to that tribe. For bonus points, make it a common.
Good luck and have fun!