Others will be able to give you better advice than I can but here's what I know will most likely be pointed out. First and foremost, the 3U is extremely small to be wanting it to be a 'stand-alone' modular. I have a Rackbrute 3U but my 'goal' with it is to use as a compliment to my Minibrute2 synth...not as a stand-alone. Because of that, I'm including lots of modulation and effects modules.
I would get rid of the 2500, it takes up a lot of HP space, which you don't have...there are probably better smaller options than this and you already have the Hysteria VCO in there. Consider adding Pamela's New Workout for a clock source and maybe FX Aid XL instead.
I'm assuming you have this set as 88HP as listed on Arturia's website...in fact, you can get 89HP which I have set both my Rackbrute 6U and 3U to.
A very clever module that @Lugia recommended for me in my 3U case is the Bastl Hendrikson which will allow me to incorporate external stompboxes for effects, etc as well. They are discontinued, but you may still be able to find out somewhere. The Hendrikson combined with this: https://www.sweetwater.com/store/detail/TriParMix--electro-harmonix-tri-parallel-mixer you'll be able to add up to three other external devices into your setup :) Other options to Hendrikson are Strymon AA.1 and Malekko SND/RTN
Maybe also consider adding ornament and crime as a swiss army knife utility module as well if you have the space.
JB