I'd really rather a smaller PMOP (If it's, say, a Commander class PMOP and Leader Ginrai, sure). The rest are also likely to be quite fun. TBH while I'm passing on the Primes out of necessity, I'm curious as to how else they can be used.
I agree that the FFOD head on Prima and the general truckyness of the altmode make U-Haul Robot a good shout.
Solus Prime, IDK. Probably a lot of retooling in any case. Animated Elita 1 would be very unexpected. I've been waiting for a Velocity for some time, this might not be a bad start. Or the Mistress of Flame (retool the hammer) or one of the other Camiens.
Micronus... Pretender Scrounge would be wild.
Maybe they can play with the reverse of the Scourge to Alpha Trion track, and make "The Burden Hardest To Bear" Matrix-corrupted Lumpy Scourge. But that seems like a waste.
A heavy retool of Quintus into a Vok (fold up into a skull shape instead of a squid shape) feels like it could be interesting, but that's a LOT to retool.
Nexus as Wing Saber, and the limbs as Prime Force drones, sounds like a silly fun time.
Alchemist... someone from the House of Ambus? The body shape makes me also think of Bulkhead or Rhinox. Cyberverse Maccadam colors are also right there I guess. Uh... Animated Sentinel Prime?
I'm likely the only one, but I still want a Flaming Hot Megatronus that doubles down on the Fallen-ness and just goes for broke with it being all black and charcoal (no purple) and giving it whatever paint treatment in orange and yellow that would really sell the walking furnace glow he has going on. And maybe a bajillion more flame effect pieces. (some third party made sweet-looking ones for SS86 Hot Rod). I agree that the upper body would work great for a broad-chested IDW Megs and the cylinder shoulders seem easy enough to make work. I've done a bit of a causal, slow re-read of the era that's making me miss the designs, so I'd like to see this tried. But the wide-set hips feel a little incongruous with Milne's art.
Onyx likewise has another color scheme (IDW i think) that I think people are more into. It's also not very "onyx" (black), being shades of blue and purple, but it is a very handsome look. Other than that, I think the Cybertron Scourge color scheme (the Cryo Scourge colors are actually similar to the IDW Onyx colors) would be great.
Amalgamous... as long as it isn't Shockwave, tbh. Maybe even give him an entirely new head thats non-humanoid in a different way.
I wonder if there'd be any interest in a Hall of the Primes box set that has all twelve as the gold statues they're depicted as on the packaging. Sort of like a Golden Lagoon thing.