Well I thought you had a lot of really fun and creative interactions and ideas in OPP so if it winds up feeling anything like those I'll be very excited to see!
I think OPP's biggest strength was it's really curated interactions, characters and scripted events. The game COULD have just been the endless mode and it would be perfectly enjoyable but it's the way the story kept building on or exploring ideas that made it so loveable to me. All that to say I guess that I think focusing on curated puzzles over randomization or making each class play fundamentally different just for the sake of it is definitely a good approach.
Also I felt a little weird talking about what kind of TF stuff I might like to see in my original message as it feels a lot like begging when at the end of the day I just hope you make whatever it is that you want to make, but whatever I'll go for it here.
Ive got a lot of stuff I would love but feel like might clash or just generally not be exactly what you're looking for like animal, monster or object tf but on thing I thought might be more in line maybe if you're interested in. I've always been a big fan of detachable or just disappearing of parts. So like armless, mouthless, ect, whether it's from an accompanied tf like a slug-girl or just on its own like magically disappearing or detachable parts / portals even like the portal bra from OPP, but with like a head for example. Or just like, a dullahan monster. Don't know if you're actually looking for these sorts of suggestions or if you'd prefer strictly mechanical / gameplay feedback, sorry if that's the case.