Honestly I'm still not 100% with this design so it may be tweaked a little in the future.

(Again you can see how I just went "fuck it" and decided for a far simpler style)

He's tired, he's gay, heeee's purple, it's Galvatroooon!

So as you may notice he's very similar to what we have seen in the movie, so I'll explain.

Galvatron originally thought Quintessa would just allow him to have his spark back but keep his rad Transformium powers, but Quintessa's a shit and decided to make a downgradehim Megatron again.

At the end of TLK Megatron was flung to Earth but still seeing him as a useful pawn Quintessa brought him to Unicron too.
Unicron accepted this offering and reverted him back to Galvatron (with a few minor changes).
Now he's working alongside Quintessa as one of Unicron's underlings only now he's a rad purple colour AND has his transformium powers back (except he appears more like fluid now.

Only it comes with specific drawbacks and a deliberate leash to bind him with, one Unicron gladly tugs on.

Anyway, Galvatron wants Cybertron good and safe with it's people prospering and Unicron is NOT the way to go, but what's a guy to do?
His life's in the shitter, one of his closest allies is senile as fuck, his brother hates him, Quintessa is still a rude duplicitous bitch, Unicrons just a nasty abusive shitbag of a boss AND TO TOP THE FUCKING CAKE OFF.... The flying guy with horns is fucking hot as hell but he's ridiculously loyal to Unicron and very flawed himself.
Oh boy.