I don't think it will be thundercracker just because of how things worked in the ongoing. He never really officially turned on Megatron, only Skywarp seemed to have any real problem with him. And it's not like Megatron and the decepticons didn't know where he was while they were all on earth.
From what we've heard in bullets and comments from Ultramagnus, it seems that they operate rather independently from the rest of decepticons and probably deal with Tyrest, UM, and the galactic council about as much as they do with Megatron or the decepticons in recent times.
My guess would be that just coming across the crew of the lost light they simply see drift as an old loose end to tie up, since his actions with lock-down in his mini. Kinda like overlord, but more officially a traitor.
I wonder if they had even heard about the after-effects from AHM and Chaos at all.
ok to print
Pirate, definitely pirate.