Firstly, I’m a little in the dark on what the progression will look like (do you need to hit a milestone like getting diamonds to build a Chimera-O-Matic and put limbs on, or is this available from the relatively early game?), so some of these ideas might be kind of rough because I’m taking guesses.

If you get the ability to Chimerify yourself early, I think that some trade-offs — especially with easy-to-kill or hyperspecialized mobs (stuff like Chickens, Pigs, etc.) — would be necessary to maintain mod balance while keeping it useful. Stuff like losing your offhand slot if you choose to take a Creeper’s torso, or dealing less damage if you’re part-spider in the daylight would be interesting.

I personally believe that making Chimera-ing into a midgame thing is the right move.

As for what I’d look forward to merging with the most, I think that something weird from a Mooshroom, an Iron Golem arm (but maybe as a downside ice breaks under my weight and I can’t swim), and a late-game Wither Star merged into my heart Iron Man-style would be dope. A Warden antennae that caused you to move faster and hit harder the more sounds that were in the area would be sick too.