I'm a big fan of the Ornament & Crime. Quite a few manufacturers make them nowadays, After Later and Cal Synth have a nice 8hp version. With stock firmware it can be a great flexible quad quantizer or a modulation source of many flavors. There's also the Hemispheres firmware that lets you run 2 different programs on each half, so you could have a 2 channel quantizer plus 2 channels of fun modulation or logic or whatever combination you want.
Pam's (New or Pro) Workshop is also great as a clock and modulation source, and can be set up as a quantizer as well.
For great modulation, Xaoc Batumi is a classic for a reason, and Behringer has their clone if you don't want to shell out for the original.
The other thing I'd very strongly recommend is a quad cascading VCA, a la the Intellijel Quad VCA or a Mutable Instruments Veils clone, and again Behringer has their version of it now too.
If you could only get two more modules, I'd say the Ornament and Crime and a quad VCA would be my recommendation.
Also if you haven't already looked up the Maths Illustrated Supplement, that will teach you a ton in the way of voltages and the many different uses for your Abacus. You'll probably want a second Abacus after that : )
Cheers and happy patching!