Personally, I'd lean more towards removing a sound source than a modulation source, given you already have a good collection of synths to begin with.
Plaits is an all-time great VCO. One could argue for days, months, years about the greatest [type of module] out there, but Plaits is a name you'll see constantly in such arguments about VCOs. So I'd say keep that. Between the Surface and Ensemble Oscillator, I'd say the surface is "more unlike" Plaits, so keeping it would allow for the greatest variety. The Ensemble does do things that Plaits doesn't, but Plaits has ways of achieving pretty similar sounds (as fundamentally, they're both oscillators). Surface works on a different principle: electrical emulation of a physical resonant cavity, so it's going to achieve sounds in a pretty different space than a more "normal" VCO.
3x MIA is a great utility module. Mixer, Attenuverter, DC-Biaser. It'll find its way into many a patch in this rack.
I'm not familiar with the 3DVCA. It's certainly interesting, and sometimes modular synth can be all about embracing the journey and seeing what esoteric lands your modules lead you into. But I'm having a hard time wrapping my head around how to actually use it. 4 Inputs/Outputs and 3 CVs...I get that it's somehow related to 3D space, but I'm just not sure how I'd operate it besides "put CVs in and get something different out". It's also pretty lacking in manual controls: only a shared "Level, Pan, Crossfade" for the entire module. If you've studied this module and think it can provide an interesting workflow for you, go for it. When I was recommending new VCAs, I was picturing something more like Mutable Instruments Blinds (a Ring Modulator) or a clone thereof (After Later Audio's Monocole is a 2-channel clone). Ring Modulators are basically VCAs that can go to negative gain. A "normal" VCA will just stick at 0V if the CV is negative, but a ring mod will invert the input signal.