A kalimba module
That would sound really nice with the Zularic Repetitor/Mutable Instruments Rings combo.
http://mutable-instruments.net/modules/rings
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.
http://www.bastl-instruments.com/modular/
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.