I would have suggested Optimal Optimus, but they made TMII Megatron too small to fight a Titan Optimal. (Optimal's size was inconsistent on the show anyway... He shrank over time)
If they revisit Optimal Optimus, I expect he would be a commander class or leader that borrows elements from the POTP desgin, thou I could be wrong
For Titan class, a possible idea for them to go with, is kind of the reverse of the Ark & Nemesis, we have the robot mode but not an alt mode and that be the Autobot that Hound used as a hologram at the end of the ep where the constructicons appeared for the first time, in Heavy Metal War:
This guy: