Unicron, Megatron to Galvatron, Quintessons and their army, Nemisis Prime, or any combination of the 4.
I say Decepticon civil war would be the best way to go for #3. Unicron would have to be ENDGAME. As in, last TF movie ever type plot. #4 could be something about bringing Earthlings to Cybertron and maybe some humans siding with the Decepticons, and #5 could be about trying to prevent the Decepticons from summoning Unicron, while movie #6 would be about trying to defeat Unicron.