Well, the Mother-32 only has saw and pulse outputs on its oscillator as far as I have read. Also... it only has one oscillator. So you might consider integrating two stand-alone oscillators that can generate those as well as triangle and maybe sine. If you do that, you will probably want a small mixer to sum them as well. I'd want FULL oscillators capable of sync. There are so many choices out there that you'll have to decide what's the priority.
Eurorack effects that can be modulated might be nice. Again, there are so many..
I feel like a broken record... but a multi-function module might also be great if you're looking to explore new functionality. An Expert Sleepers Disting EX has a nice selection of functionality. If you find a function that you constantly use, then it might be prudent to buy a module dedicated to that functionality. For example if you're constantly using the quantizer, then you might want to buy a dedicated quantizer.