Reportedly it's in pre-production, confirmed by Cranston.

Rumors are currently that the film will reveal what happened to Jessie after he was rescued by Walt in the finale, possibly his redemption. I'm guessing we'll probably at least see Skinny Pete and Badger again, and maybe the Whites (Perhaps even Walt himself, in flashback?)

We sort of have seen what's happening "now" with Jimmy/Saul/"Gene" in the black and white mini-episodes that open every Better Call Saul season. Still I suppose it's possible he could appear somehow (also via Flashback; heck Better Call Saul is pretty much a flashback already).