If they want to introduce Unicron to TFP, how would his presence affect the series? Would Megatron really side witht he Chaos Bringer/Dark God? How would the Autobots fight against him? Seeking out the Matrix of Leadership and use it to defeat Unicron like they did in G1? Would the Decepticons and Autobots work together to fight Unicron if his goals prove to be not in favor of Decepticons?
I doubt Megatron would want to SIDE WITH Unicron, as he's more for conquest and gaining an Empire instead of simply destroying everything. Buuuut I do think Megatron WOULD want to gain Unicron's power, while Megs may not want to destroy everything, I think he'd like to have the OPTION. Strictly as a guess, Megatron'd probably find a way to become more powerful than Unicron, possibly by siding with him temporarily to learn and scheme, or by staying out of the fight between Unicron and the Autobots while he plans.
Then again, would Megatron be capable to resist the influence of Unicron after selling his spark to him (he accepted his blood aka dark energon by his own free will)?
Also, I've been thinking. Who's to say Unicron WILL be a giant planet-sized Transformer in Prime? The creators could change that.
No one, but in what continuity is Unicron not planet-sized? And what's up with the Transformers: Prime's crew alluding to Unicron's inevitable appearance with showing planets whipping by stars and planets? There is no greater threat than making Unicron larger than life.
My point is, the producers could come up with some spin on Unicron that would blow both our minds and anything we're speculating.