I agree with @Sweelinck, particularly about watching that video. If you haven't seen them already, Mylarmelodies' videos are immensely helpful in putting together a rack and just understanding modular in general.
As for your case, my first thought is the QPas is taking up a whole lot of territory. It may sound great, but if you can't fit anything else in the rack to use with it, it won't work for you the way you want it to. To be honest, Plaits is so robust and flexible (particularly with the new firmware update) that you could probably get away without a dedicated VCF.
Next, you need modulators. Things like lfos that will give movement and flow to your patches and let you change parameters without futzing with tiny knobs like on the 2hp modules. 2hp makes great modules, but you really need to think about usability and spacing with their knobs. For example, would you be able to play with the Mix's knobs during a live set without accidentally messing with Plaits' settings? Back to modulators, one great module is DivKid's ochd, 8 triangle lfo's in 4hp, it makes really organic motion. Doesn't help with other wave shapes, but I love it. Though it's a bigger footprint (10hp), Xaoc's Batumi is a classic and gives you access to a wide range of lfo options. Amazing when self-patched, too.
How about percussion? It doesn't usually makes sense to waste space with drum modules if you don't have much hp to spare (use outboard gear for that), but if you wanted everything on rack, something like Erica Synths Pico Drums (3hp) might be interesting. If you are using off rack percussion, or just because it's awesome, then something that can send out triggers would be helpful. You could use that to build patterns or any number of other useful things. Of course the Pamela's New Workout / Pro (10hp) is a standard, but there are other, interesting ones like Euclidian Circles by vpme.de (10hp) or various logic modules (which, in fact, Pamela's can also do).
Finally, but maybe most importantly (@JimHowell1970): utilities. Your case already comes with a good starter pack (attenuators, mults, vcas, mixers, Maths), but that's really only scratching the surface of what's out there. You can squeeze so, so much life out of your system using utilities. Also, they're often cheaper than the sexier vcos and vcfs, so that's a plus. Something like Ornament and Crime (particularly with hemisphere loaded) gives you a huge number of utilities and therefore flexibility in something like 8hp, depending on who makes it. I haven't yet used it myself, but Expert Sleepers' Super Disting EX Plus Alpha is a monster for utilities (also 8hp). While they're not very fun to play and do have some menu diving, either one will open a ton of doors to different tools that will change your modular experience for the better.
Hope that helps, and good luck!
(edited 'cuz I thought of more things to say)