A bipolar-valued MIDI-to-multi-CV interface / multitrack sequencer / programmer (i.e., a Pro-2 minus the sound generation facilities) would do the trick, with each section performing a useful feature that would apply not just to Eurorack-format modules, but also SEMs, 0-Coasts and the like. (Of course, its usefulness assumes modules which could accept multiple linear or exponential modulation values.)
I'd also love to see a good-quality, sensibly-engineered polyphonic voice assign module, maybe with splits, velocity, etc.