Monomachine 2.0 Keys with a Vocoder and revamped frequency modulation
Or a combination circuitry and digital video synth/sampler would be way cool fun.
Enjoyably integrated and emotionally relevant Audiovisuals are the future of entertainment.
Olfactory synthesizers might start happening in 50 years if clubs install perfume generators.
Holograms would be cool but that most likely requires the technology of giving photons mass and intelligent responsiveness.