@Lugia undoubtably knows more than me, but from recent explorations of my own I think it's possible to build at least a generative melody box in 62HP, so I took a crack at it, see below:

ModularGrid Rack

I left the Wogglebug and the Marbles, they seem great, but pulled out everything else. We're coasting a lot on the power of Marbles (for example we don't need a quantizer, it can do that for us), but that's ok, it's a powerful module! So, to review:

1) Shifty lets you route the output of one of Marble's melodies to up to four different synths, and has gate triggers which can be used for envelopes and/or for manipulating other parts of this rack. Want two synths to add some harmony while others handle the melodies? Shifty can help out here.
2) Switchblade lets you toggle different inputs, for example want to alternate the Deja Vu input into Marbles from an LFO or a Sample and Hold? Switchblade can help you there.
3) Ochd provides 8 long running LFOs, that can slowly shift different CVs around your system. Want long, slow change over time alongside a little more busy-ness? Ochd is your module.
4) Sample and Hold is pretty useful for generative work, RND Step gives you 3 sample and holds normalized and with pink noise sources if you don't patch them. This is a great control tool and a random source alongside Wobblebug.
5) Adding a Slew Limiter gives you a lot of new terrain to explore, so I added a Time Warp. Using this you can for example have the gate triggers from Marbles turn into smooth, rolling CVs that will stay high if there's lots of notes and will drop down if Marbles quiets down. It can also be used as an envelope generator, triggered by gates around the system. Even better, it's gate-able itself, so other modules can control when WMD fires off.
6) Speaking of gates, the Rotating Clock Divider lets you take one gate and create rhythms around it. If we're making ambient rhythm might not be the first thing that comes to mind, but this will help take output from e.g. Shifty/Marbles and create a sense of movement or complexity around the system, triggering an envelope to adjust a filter somewhere every so often, or toggle a Switchblade switch, stuff like that.
7) Hyrlo, smaller 6 channel mixer, will help you collate all your synths together.
8) We've gotta have reverb for generative ambient, FX Aid does a ton on top of that so it's a no-brainer in a rack this size.

For the 1U tiles, I added
1) a Quadratt so you can attenuate different control signals
2) a dual VCA to make more complex control signals
3) an oscilloscope so you can understand what's going on (surprisingly important!)
4) an LGP to let you do a little signal volume control here in case one of your outboard synths doesn't have VCA control.

One important note: I only picked modules that I felt you would still want were you to expand down the road. Everything here is small, yes, but also powerful and could hold its own in a rack of any size. So if you start here and find you really do want that 2 x 104HP rack you can just port some of this over.

Anyway, that's a lot of text, hopefully some of it's useful!