leave a blank for some time while you figure out more about what feels missing to you. After all, that's often a good thing to do if you're in doubt, it's free and modular is not a race, you don't have to "finish your rack".
It's your custom instrument, so take your time to think about how you want it to be ;)
-- toodee
I like this advice, there is no rush to fill spare space, let it breath a while and you might just figure out what you need versus what would look nice in the gap ;-)
For me prsonally I'm at the state of slight mental anguish as to whether I should get a Shackmat Gemini's Path, a Ripples v2 Clone or a O_C, they are all viable choices, some sensible and some fun. Sometimes its better to get the 'Bread & butter' elememnts before going for that fancy Frnch Patisserie.
Thats me though and I have 3 disinct paths I wish to take my modular, 1/ Main Case whih is mainly for generative, 2/ My Percussion and Acid rows, then 3/ I'm planning a seperate weird, sound mangling esoteric rack.
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.