Newsarama has the cover to Teen Titans #99, revealing that Bart Allen’s “dark twin” Inertia returns as a member of Superboy-Prime’s new Legion of Doom.
Inertia was created as a clone of Bart Allen and sent back in time to replace him (several issues of Impulse, then the “Mercury Falling” storyline). He later tricked the Rogues into killing Bart shortly after he became The Flash (“Full Throttle”), putting them through hell, and they got back at Inertia by killing him in Final Crisis: Rogues’ Revenge.
No word on whether Inertia is resurrected, brought in from an alternate reality, or a younger version brought “back” through time travel.