If there are certain songs you would like to play in Rock Band 4, Harmonix is taking requests.
Granted, not all requests will make it in, but if enough people request a specific song, it could make it in providing licensing can be arranged.
I’ve already submitted around 20 myself. Poor Harmonix.
The developer plans to support older instrument controller with the PlayStation 4 and Xbox One version, but new controller will be offered as well. Just don’t expect them to include any new bells and whistles.