It would be pretty cool to have a multi-band compressor for bass (and I suppose it could be cool on guitar too). Even a two-band comp would add a lot of versatility with a full set of parameters for each band. This can be done currently with parallel path IIRC, but having multi-band without having to split the path and add HPF's and LPF's would be really nice. For example, having a mix parameter on the distortions makes it much quicker to dial up a bass distortion since splitting the signal usually isn't required anymore.
My ideal compressor would have a crossover frequency parameter and a link button to link the high/low parameters for initial tweaking.