Envelopes.
I really like the Tiptop Buchla 281t because it has 4 loopable envelopes with CV control over attack and decay plus a quadrature mode that kind of makes them all loop and interrelate in weird ways. You might like it in an ambient case as a sort of freeform brain/hub for modulation and clock - I tend to use it both as 4 individual envelopes and also as a weird freeform sequencer of sorts. It's a big module, but it looks like you already have some spots where you've got some cramped modules.
But whether or not you take my specific suggestion, I would just say look at envelopes that you can play around with by hand in contrast to the Zadar, which is awesome but not quite as hands-on, for some variety. I wouldn't put any more small little modules in there unless you find one that really speaks to you, and I'd probably even start looking at blank panels to put in between some of those things at the top.
All of this is sort of beside the point, though: what do you yourself feel is missing? Do you like how it feels so far? Is there anything you don't use as much as expected? Do you like the Forbidden Planet filter? There's always another filter to try out.