A couple of thoughts. Coz i prefer use VSTi instead of sample instruments (say use renoise sample engine as advanced drum machine), the new version of sampler/plugin/midi panel not so logically as previous. In previous version it was very clear to bear in mind about instrument control (from left to right, with folding IN and OUT) MIDI IN -> INSTRUMENT itself -> OUTPUT BUSES. Like FX chains. Now it’s not so clear. Anyway it’s very good news about renoise sampling engine (and flexibility of this engine), but works with tabs SAMPLER/PLUGIN/MIDI in up2down style instead of left2right (as it works in previous version) is less intuitive i think. Also there another thing happens - i cannot see my pattern when i need to load/save VSTi program/bank (in case of one monitor configuration).
IMHO the top left corner of app is overfill with controls. There is menus, main control keys (withous any box around them or some sort of divider between menu and keys), 5 tabs (edit/mix/sampler/plugin/midi) and some misc controls. I spend a cople of seconds to find main control keys in new version on first run 
May be control keys looks like BPM/LPB/OCT boxes. They all have approx the same height. In sequencers like Presons studio one/Ableton live the main control buttons shifted to center (the previous version of renoise works like this). In Logic audio - controls placed in top left corner, but it based on contrast background. In cubase - controls are floating window. May be is possible to accentuate the main keys with something? Anyway it my opinion only.
And (as i wrote a some time ago) another suggestion - please, make values of dB in master level indicator on top of the renoise window Coz now not informative (may be use only for check overflow status) and i always use Sonalksis freeG plugin on master to check dB within mixing process. Thank you!
And anyway - great thanks for great job!!!