I think about starting by adding a full voice, a Lifeforms SV1. (my first idea was a Plaits but i like classical osc sounds and i want it to have its own enveloppe and filter which would mean adding others modules instead of using the EXT IN of the MB2S, ..... so a flexible full synth looks better for me.... at first)
And a multitap delay (i don't know which one. Magneto seems to be the best but it takes a lot of space.) I've seen several people combining 2 Milky Ways (?) I want "stereo" outs, mixing, ping pong, etc.
And a Mutable Links because of its precision adder : my goal would be to add a Sequencer which primary function would be to transpose the MB2s sequencer. But i don't know which one. Probably a simple and not too wide one.
(At first the Quad VCA may be a little useless but i think it will be later)
(and maybe a sampler, i've always been fascinated by the Emax EMU and Depeche Mode early area)