It would be very useful to be able to loop the phrase on all channels from inside the phrase editor.
Now composing polyphonic parts with long chains is kind of slow, when after every edit one needs to listen (b+right) from the beginning of the chain.
Also, for sample mangling an option to slide loop points would be cool.
Like, for example setting initially an extremely short loop, and then have for example LESL and LBSL commands (for end and beginning loop point) with parameters for speed (like with other slideable parameters, change at 7f for forward, backward) and distance in 256ths of the sample.
You can try this in the Renoise sample editor by just dragging the loop points around when a sample is playing, the effect is quite distinct.
instrument sync:
This would work mainly with short samples, especially single cycle waveforms. Like a control voltage oscillator sync, but the slaved instrument is instead restarted each time the master instruments loop reaches the start point. I can hear this now in my ears, and it also sounds like not such a CPU intensive action. Some logic would have to be thought out for cases where the master oscillator is playing on several channels.