I'm going to cheat somewhat by grouping features together, and also I think mention things no-one else has:
1 - Better plug-in compatability.
- A DirectX BitBridge or equivalent to help people (me!) migrate to 64 bit more easily
- Versioning the VST Scanner so that an update to the scanner doesn't break existing plug-ins with older versions of SONAR
- Better VST compatability across the board (SONAR isn't the best in this department)
- Proper mono support (good for UADs etc)
2 - Performance. Although I don't think it's as critical as it used to be, SONAR does not perform well compared with other DAWs, especially at low latency. In particular, almost all have some kind of way of improving performance when low latency is not required (eg. For input monitoring) - ASIO Guard, Hybrid Engine etc.
3 - GUI improvments. For me, there are 2 big ones needed: smaller widgets (we've been losing track data space for multiple versions) and a revisiting of the customisability of tools as we had in 8.x (the PRV tool customisation was unbelievably good and knocked any other customisation I've seen in any application for six*).
* Obligatory English reference :-)