If we're going on size wouldn't part 3 be Planet Wars?

Unicron vs Primus with I'm guessing lots of add-ons (a-la mini-con style) to kit out your planet?

I'm not adverse to the idea of Beast Wars as i'd really like a dinobot figure at some point but I'm not convinced they would re-visit it with the vigor they did with G1 in Combiner Wars.