Rack in the image is a little misleading; good thing I checked the module listings for hp instead of going by 'eyeball', because that's not 84 hp wide.
OK...top row has 14 hp left (5, 45, 20 present), and if the goal here is a lot of weird crossmod and strange behavior, my suggestion would be some sort of module that can be a 'hub' for all of the in-cab voices at the same time: Abstract Data's ADE-32 Octocontroller. Check the listing...I think it would make for a good 'hub of weird behavior' for something along these lines. On the bottom, you've got 2 hp left, and my suggestion there would actually be two extra linear VCAs (by, who else, 2hp) to tandem with the Octocontroller to add some further levelling oddness to some control paths. Careful with the external mixer, btw...make sure its line inputs have both the extra headroom AND attenuation to deal with the synth-level signals.