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!
-- Lugia
Wow, thanks for the suggestions. I was trying to convince myself to live without the NIN (to save that little space), but yes, I'll end with one soon (hating the odd hp modules, pico DSP was there to make it even again ).
Also, I've been reading about a Intellijel little 1U 8hp VCA coming soon ( https://www.modulargrid.net/e/intellijel-1u-dual-stereo-vca-module ) that little module could be worth the sacrifice of the 2xUSB 1U power plugs.
Finally the other option is to just use the pedals+guitar with the 1/4" jacks on the top of the case and audio I/O 1U and take another big VCA module instead (but not sure wich one for 11hp).
P.D.: My wife says that you are a mystical beast