One of the more interesting filters on the Korg Z1 (among others) is a dual bandpass filter, which is, simply the audio signal split up into two resonant band pass filters before being joined back together for the output. This allows for some pretty interesting formant style effects. The way the Z1 handles simultaneous modulation of both band frequencies is to add a value to both in this way:
If band 1 is at 25% and band 2 is at 75%
Overall frequency controller at 0%
Band 1 is at 0hz
Band 2 is at 0hz
Overall frequency controller at 50%
Band 1 is at 25%
Band 2 is at 75%
Overall frequency controller at 50%
Band 1 is at 100%
Band 2 is at 100%
Having to do this kind of filtering, currently, as you need to split the signal into two sends each with its individual filter, modulate that filter and then pipe them both back into another send, is really, really awkward. Being able to tweak a single parameter for this kind of filtering is fantastically useful.
So, just a thought off the top of my head; It’d be cool to have a multiple bandpass filter with a flexible number of bands and a unified frequency/resonance controller for the lot of them. It’s hardly critical; Plenty filter vsts solve this for you. But hey! 
Put a simple demo of this technique up at http://www.doomsday.no/music/xrns/2bp.xrns
