I've been working with a bunch of difficult-to-sample MIDI hardware lately (e.g. paraphonic synths or unsynced LFOs) and noticed how awesome it would be to have the ability to apply instrument modulation to MIDI CC values. Since every instrument already has an associated MIDI-Out, it would by directly applied without the need for setting up instrument-specific doofers on a mixer channel. Since it's possible to put a Line-In effect on the sampler itself, we'd end up with a highly practical way of packaging control over external hardware (just load/save the instrument).
In the same vein, it'd also tremendously help to be able to feed the sampler a Line-In signal. Some synths just don't come with the modulation you want, but Renoise already knows about whether a key is played or not. Think about the possibilities! I'm dying to see my Waldorf Rocket have a proper Attack value to it's envelope - post modulation would do the trick for most situations. Not to mention sound sources without envelopes (e.g. modular equipment).
The two might be tricky to get in due to latency and processing order, but it'd be fantastic to have! A massive quality-of-life boost for everyone who mixes hardware and software.