well any of them that is possible. if a knob can be twisted and an effect achieved, it would be nice to point a matrix slot there. but specifically, it would be great to modulate the wrap drive, tone and gain to add some character to a patch (because im unable to get that distortion to fade out even after tweaking the amp envs). but every filter parameter deserves it honestly. i feel like its a missed opportunity to make preenfm a more current sounding synth. i often want to dynamically tame a sound after it is distorted. the formant filter deserves parameters as destinations too. most of the subtractive filters would be a good choice too.