It is 2 in the morning and I am still not done.
Most continuities have Liege Maximo and Megatronus being evil primes, though that is different Transformers One. I personally have it in my continuity that Megatronus is a good person and Liege Maximo is the only evil one, but regardless, I imagine Primes who use their Primehood for evil are more imprisoned in the Matrix rather than vibing. They can't possess the user.
However, if there was a plot point of an evil Prime (Maximo or Megatronus, whichever you prefer) breaking their imprisonment and actually possessing Optimus?
I think it would be a great showing of just how much Optimus usually holds back because I 100% believe an evil Optimus would be more terrifying than any Decepticon. He would be terrifying the Decepticons themselves.
Optimus possessed by an evil entity and going on a rampage and wreaking havoc and actually forcing the Autobots and Decepticons to team up so they can restrain him and figure out to reverse this possession while the Primes in the Matrix are also fighting to try and pull the evil Prime back into the Matrix so they can be imprisoned.
Idk, give me versions of Optimus fucking losing it or turning evil or being possessed and make him be so terrifying even Megatron is like "can we have the good one back?"