I love the idea about instrument patterns. Renoise becoming a phrase sampler and sequencer rolled into one 
So you’d have a limited subset of features from the pattern editor? A lot of thought need to go into that one.
For instance, I would expect to be able to play an instrument with an arpeggiator (standard up/down/random walk mode, and the ability to combine the notes being played). This is not possible in realtime, not even with the Live-mode that pysj suggest (which is another great idea btw)
Finally, don’t forget the existing instrument annoyances/shortcomings (yes, I keep track of such things):
The ability to control an instrument thru a specific MIDI channel
This is really not useful the way things work ATM. For live performance, one wants to be able to change an instrument while playing (and not having to fiddle with mouse + keyboard, or even looking at the screen). Another practical use is when using an external sequencer to feed MIDI to Renoise.
The ability to specify “auto note off”
See this thread for the description
Shared samplepool
Microtuner generates huge files 
Auto-select sample split
Forgot this one. Being to select a split by hitting the corresponding note on the keyboard. Can’t remember who suggested it, instrument-building would become a lot faster.
PS: I had an idea a while back, about how advanced routing could be displayed, which easily could be applied to instruments. Complex routing in renoise.gif. This type of diagram has a type of flow, which is pretty easy to interpret. And fit’s well with the Renoise aesthetic
