More VCAs. You have plenty of modulation sources, but you only have VCAs that seem to be for your audio. Being able to put your LFOs, looping EGs, and so forth under VCA level control will add a lot of nuance to your modulation use. But go smaller, something along the lines of Happy Nerding's 3xVCA or Intellijel's uVCA II (half of a Quad VCA, basically) would work wonders in limited space...both are just 6 hp.
DO add the Zadar's expander module, the Nin. Given that the Zadar is intended for modulation uses, the Nin adds some basic functions that make it work nicer for that sort of thing.
So...that leaves 5 hp. Given that you've got loads of stompboxes at hand, perhaps a send/return module would be useful in that space. And with 5 hp, Bastl's Hendrikson seems like the right choice. It also gives you CV over your wet/dry mix level, can function as a feedback controller, has clip indicators for your I/O signals, and your external loop I/O points are proper 1/4" jacks for directly connecting typical guitar cables...no messing with fiddly 3.5mm adaptors here!