Maths is a brilliant starter module - google the "Maths illlustrated manual" - which has illustrations of 32 self patching programs for maths - which is a great primer not only for Maths, but also for patching modular synths in general - it would be top of my list for adding more functionality to semi-modular synths
A quad cascading vca, such as Veils, would be second on that list
I'd probably replace quadrax with one of the effects though - either fx aid xl or warps
I'd also recommend grabbing a few stackcables - being able to use the same signal a couple of times is really handy especially in a small system - and they don't take up rack space
the next modules on my list would be Kinks and Shades or similar - they both add extremely useful functionality
Thanks for the feedback @JimHowell1970 - I just placed my order for the Maths last night and will be ordering again the first weekend in June to get the Veils...sounds like you're recommending to get either fx aid xl or warps instead of the Quadrax, not sure I understand completely what the Quadrax brings to the table, but I'll read up some more on it...again, my idea is to build what Lugia has recommended but do so gradually with what makes sense to start with...thanks for your recommendation!
JB