at a bare minimum, you'd need expert sleepers if you want to use it for midi, and as far as i know, use that to modulate the rest of your system. so you have your trigger & oscillator (a second would be a good upgrade for thicker tones). then, for a bare bones synthesizer voice, you should have a VCF to shape your tone. if you'd like really flexible one, look for a multi-mode one like intellijel's popular polaris which would add thick, juicy, rubbery, some vocal & even grungy tones to your system. filters are almost more important than oscillators for shaping your voices, so a flexible multimode one is a good choice. if you just want to get started right away and don't care if your tone palette is limited, you might get a combination VCA/VCF like ryo's flexible apature which isn't as flexible as the polaris for filtering, but does offer some VCA distortion potential to your rack along with coveted vactrols which are good for making percussive sounds. finally you need an envelope generator. you could use maths for that and a bunch of other utilities, but a 4 stage ADSR will give you more control over your envelopes and they come in packages as small as 2hp, but a cheap doepfer 8hp would be more comfortable to use. for a small system, a polaris, aparture ADSR combo would be pretty powerful. i'd say that's a bare useable minimum mono synth setup.
it would help a lot though if you had an LFO (some would call an LFO essential), a clock modulator, a noise source, some kind of sequencer, depending on what you're doing, a buffered mult for splitting your patches to different modules, and CV modulation. an intellijel triatt would be good for that as it can double as a mixer which is handy. also, you can get modules, like maths, that do a bunch of different things, but to me, they make things more complicated and it's easier to lose functions because you can't use EVERY function a module does at the same time.
a bare bones system though is control/modulation (midi/sequencer/keyboard), an oscillator, a filter, an envelope generator & and a VCA, but you're limited to what you can do. that's a basic monophonic synthesizer aka "voice".