I start with:
Open DSP plugin C++ API for third party developers. Access to all the GUI features in the API.
Javascript / HTML5 integration, next to LUA. (Maybe webkit integration for gui extensions?) Very fast open source javascript interpreters are available. JS has the following benefits: 1. Gigantic amount of free libraries, including gui stuff 2. Common standard in lot of applications 3. Very simple and beautiful because of ECMA standard 4. HTML5 guis can be easily built with lots of tools 5. Gigantic size of community 6. Google and Mozilla already developed audio/dsp manipulation for JS
Real freeze functionality, targeting especially external synth/audio generator devices. Like Freezing pre-fader/post-fader options, automatic RE-freeze on change (the note data could be still editable)
Displaying note lengths as blocks instead with a note-off, easy resizable/movable by using mouse (a.k.a. moving a resizable block)
Smooth vertical scrolling by real required amount of pixels, not by pattern line. Position bar would be then a line, not a bar anymore.
EQs editable directly in the spectrum view, with 12/24/48hp/lp and other types of filters.
Intelligent phrases just like good arpeggiators (or Karma engine on Korg synths), which adapt to multi-pressed keys/notes
LUA protoplug VST integrated natively as “DSP formula device” or “formula device 2”. Of course optional javascript would be cool, too
https://forum.renoise.com/t/new-tool-3-0-request-web-server-library-for-renoise/43274
Something like HTML integration