Possibly a big spoiler there in the trailer.
It occurred to me a while ago that one way the producers might deal with Firestorm's future given Victor Garber's departure would be to replace Garber with Graeme McComb as Stein's younger self, either through time travel or some sort of de-aging. And now we apparently get McComb's young Martin back again right after losing Garber. So is this setting him up to be a new regular? As if the Legends weren't already playing fast and loose with causality by keeping Amaya aboard...
Having a younger Professor Stein could have a negative impact on his future or the present, especially if the Legends aren't careful. Then again, this is Arrowverse and so many dead characters have been brought back to life via time travel, some of whom were unnecessary. I'm looking at you, Damien Dahrk.