I would be very happy if Doom made his debut in the periphery of another movie.Yeah -- it seems more likely they'd build up to that as they did with Thanos. Establish him in an FF film and maybe in a background role in some other films, showing him as a world leader of growing influence.
The old rumour that he’d debut in a cameo as a peer of T’Challa in a Black Panther movie was something I liked.