I'd say there is no point wishing anything for Rock Band 4 anymore.
my only hope for RB on new consoles is that the new consoles will be backwards-compatible with PS4 games/hardware.
Our only chance is for a Rock Band 5. New consoles coming out probably at the end of 2020. All of us plus many people who stopped playing would buy that. And that is the time they would get new instruments.
They will probably just try and keep RB 4 on life support until RB 5 comes out.
Yeah, all games playable on PS4 fully playable on PS5 and all games playable on XB1 fully playable on the next gen Xbox would be the best realistic outcome for next gen consoles. Not the XB1 type of backwards compatibility where RB4 and other games which use other controllers would be left behind.