It is the year 1969. The Apollo 11 mission has entered space and is preparing to land on the moon. The astronauts land, and discover a base of (your choice of characters - cross-faction, single-faction, whatever!) on the moon. What happens then? Continuity and crossover or crossovers left to author.
Ever notice that the Decepticon symbol has a chevron? Well, one of the Autobots has, and is now trying to convince everyone he comes across that chevrons add to a mech's "menacing factor." (Hence why Ratchet is so feared)