For me, I want:
Ability to set the speed that the filter kicks in from really fast to quite slow and set the range of the filter.
Also, it would be cool to have one that can react differently for different pitches/volumes of note... so it might be really fast for high, quiet notes or slow for low, loud ones.
I Like to blend the effect in with a blend looper so I can keep some upper harmonic detail in there but most filters invert the signal so you get phase cancellation (you can ask for the WMD Fatman to be non-inverting which is cool). I think they should come with their own blend maybe even with an EQ for the un-effected signal so you can say which bit you want to keep.
An effects loop, maybe with a blend is a good idea (an effects loop can also be used to trigger the filter externally by using it as an input with the usual input for the triggering).
If I made pedals, I would make a number of filters, each meant for a separate thing, rather than one that tries to do it all (unless it is digital or has digital control).
Maybe a built in compressor that is good for taming some of the wild spikes that these effects would also be a good idea? (a three band compressor?).
Maybe a modular system could work? One for controlling the envelope and another for doing the filter and another for taming the signal.
There is an infinite amount of stuff you could want from a filter, but by the time it starts turning into a synth, you start to need the pre-sets of a synth.