For all exposed parameters, a checkbox (plus a toggle for “randomize selected” & “randomize unselected”) and an “amount” slider, also a “master amount” slider (amount in this context meaning the amount of randomness, you’d prolly call it differently but I’m about to hit the bed so…). That might be overkill, but at least a way to keep it from changing some parameters (obviously different for each VST(i)) would make worlds of difference. Allowing even more tweaking is of course always nice, but wouldn’t be as essential IMHO.
Oh, and it would rule if Renoise could remember the parameter mask (not necessarily the amounts) across all instances of the effect/instrument… because the main use I have in mind would be to forbid the random button to meddle with pich/finetune or any other parameters that make using the random function not really useful with some VST(i). You’d more or less set that up only once per VST(i). Oohhh… TWO separate masks would be great, so that you can mark the parameters you really never want to change, and can set up a second (temporary) mask to play with just a bunch of parameters…
I hope that makes any sense to anybody, if not I’ll be happy to explain it better tomorrow…