Well, the main thing for me would be improved voice guide, since I am also blind.
But aside from that, isn't it time to upgrade the sound effects? There are some GM sound effects, but last time I checked, they sounded no better than those on the PSR-E series. There are also several instruments where no S.Art or S.Art2 voice is available. Banjo, accordions, bandoneon, solo cello, viola and contrabass, pan flutes, ocarinas and recorders, melodica... we may also need more drum kits, some realistic ones in diverse genres, perhaps updated brush stroke things. Some of the drum kits are a bit too clean. What if I want to play some old pop thing? Also, 1000 more jazz styles, particularly orchestral jazz like some of those new ones on the Genos 2, as well as latin jazz. And how about sidechaining for the kick in the dance styles? Improved Vocoder that sounds more like a TalkBox, and so on. Oh yes, also more free styles, HipHop styles and DJ styles. A drum n bass DJ style would be nice.