Yep, love the Quad also increases function density, which is critical if you're doing a 3u skiff like this. My advice for going further would be to jam maximum function into smaller space (hence the Quad VCA) and save the large hp spans for very specific functions that are must-haves.

As to a filter that fits this bill, my suggestion would be a Doepfer A-121-2 followed by a 2hp Mixer. That way, you get the multimode filter, but with the added mixer, you can combine response outputs for some elaborate sound manipulation, and the whole mess fits in only 10 hp.

But go small and powerful...there's quad LFOs that fit into that Dual LFO's space or less, and those would be better options with the limitation the skiff causes, and so on. Also, see if you can locate a powered skiff, such as the Erica 84hp skiff or MakeNoise's powered 104hp. That way, you take the power off the patchpanel and gain another 4 hp. The more space you can free = more space for possible functions.