A filter bank is always nice. The Serge ResEQ is popular and has a lot of character and charm.
A simple ring modulator is also a good investment.
As for filters, definitely get one with enough room to have fun - this is one of those things you'll always fiddle with and it needs to be accessible. Personally, I'd look at a dual/stereo one or even one with more resonant peaks (like the QPAS or the Vostok Atlas), but go with your heart on this one - listen to tons of examples and do absolutely play some at stores. Another option is the Instruo Traigh, which isn't necessarily cheap but features a three input mixer and a nice classic ladder filter sound. A used one will probably be a decent price.
One thing I like to use a lot for effects is a Moog Mavis synthesizer, which can be taken out of its enclosure and mounted in a Eurorack case. For a fairly decent price you get an iconic filter, a wavefolder, a sample and hold that can be used for a bitcrushing/downsampling effect, an oscillator/LFO combo with crazy range that can be used for either thingfor built in normalized FM and other cool modulation (or just running some nice Moog voices under what you do), as well as built in mixing, multing, and an attenuator. The keyboard isn't ideal for playing, but the CV from it can be routed to other sources so it can be used to trigger events in your modular system apart from regular notes. If you had specifically asked for a synth I may not have recommended it specificslly, but it's a great package for this specific purpose as well as being a very good synth if you decide you want to fold that in as well.