Yeah, I'm liking the make-our-own-timeline thing.
We could take the Aligned Continuity's Ark thing, where the Autobots launched a max exodus to evacuate all of their forces off of Cybertron. Since we don't know how many Arks made it past the Decepticon Blockade, a lot of OC Characters could come from Arks that made it past. Optimus then says that Earth is a safe refuge for all Autobots, Autobots go to Earth, Decepticons come around and now Earth as a new target. So the Autobots have to defend Earth, as well as themselves. Sound about good?
As for who I want to play, I just wanted to make an OC Autobot, we can have canon characters too.