[Matrixbeast]

To rip straight from the KR thread...
Let's be real. It'd be RID2015 and G1. As if they'd bring back anything in between

Though yes, I think it would be a pretty cool idea, and I'd totally be down for a massive TF crossover. It would be hell if they just brought back Primes and Bees/Kid appeal characters, so they'd have to have some sort of crazy balance.

A big hurdle would also be the cast sizes for G1 and the Unicron Trilogy series. Energon and Cybertron in particular had gigantic casts.
Any of those 3 series could easily eclipse the theoretical special into "G1 Returns! Feat. RID2015 and BW"