I'm new at this but I've put together the beginnings of a 104HP rack. It currently has the 4MS Row Power 30; the Moog Mother 32; the MusicThing Turing Machine; a Synthrotek passive DIY mult; and a Befaco Output V3 DIY.

I know I need multiple new, additional modules, but what I'd like your opinion on is: what would be a good NEXT module?

That would depend on what you're trying to do, musically. Right now, you have a fairly 'general purpose' module set. Before going nuts with the Magic Plastic, though, you'd better take some time to sort out where you want to go, musically, and how you think you can best get there. What musical elements do you prefer to work with? What sort of sound are you aiming for? What's your creative 'comfort zone'? Actually, these should've been on your mind when getting the initial setup, along with the question of WHY is it necessary to make use of a modular rig in your work?