A kalimba module
That would sound really nice with the Zularic Repetitor/Mutable Instruments Rings combo.
Although you could probably easily reproduce the Zularic patterns using Teletype.
Bastl Instruments has some lovely stuff, including modules for activating solenoids, so you can modularize acoustic instruments.
In a completely different direction, the 4MS Spectral Multiband Resonator is capable of all sorts of insanity.
I’ve already got plans for two cases, even though I have yet to assemble my first DIY module. I’m going to try to DIY as much as I can for two reasons: I’d like to design my own modules eventually, and I’m going to have to make at least a minimal effort to keep costs down, because this shit is addictive.