I have pondered this exact question myself, but it appears from a slightly different angle. The design of this instrument is, unsurprisingly, brilliant. Would I love being able to parallel or series either low pass with the state variable? Would I love duophonic to go with that? Would I love a couple more CV ins or at least outs? Yes to all, but the immediacy of the layout and current feature set makes those wishes and not must haves.
The VCOs, filters, mod sources, easy and flexible routing, and effects are all very nice. However, for me it is the sequencer (along with the CV I/O) that elevates the Pro 3 above any other mono design available today at any price. I may well be the only one on this whole forum to feel this way, but I have zero interest in any polyphonic synth. (I do not consider "string machines" to be synths in this case)
So, as I have thought on this (and kept my wife from knowing of such thoughts) it comes down to what CV "friendly" monosynth would I add to a Pro 3? The list so far is Dreadbox Erebus, Dreadbox Nyx, Behringer Neutron, and Behringer K-2. What other mono modules are there with comparable CV I/O?