Oh, believe me, I didn't forget that. "How's Annie? HOW'S ANNIE?!" What a depressing end...
Still, whenever BOB took possession of someone, there'd still be moments when his spirit-form (a.k.a. Frank Silva) would be visible. Like, when Laura's mother sees him at the foot of Laura's bed, or when Cooper goes to the Black Lodge, etc. BOB remains pretty much the same in all of that. So, I think they're gonna have to address why BOB -- if seen in spiritual form -- doesn't look like BOB. Or, for that matter, why anyone else in the Black Lodge (e.g. Wyndom Earle, Leland Palmer, Carolyn Earle, Laura Palmer, MIKE, The Arm/Dwarf/Little Man From Another Place) looks 25 years older.
I suppose you're right that BOB as a concept will appear, but BOB won't look like BOB did. Hmm. Maybe they'll explain it by suggesting that Cooper has been engaged in a 25-year battle for control of himself, and that the battle has naturally changed him and BOB in his spirit form. So, like, BOB possessed Cooper, but he was also almost a prisoner of Cooper maybe.