Reading through all the great ideas here, I thought of one thing I haven’t seen yet: How about an arbitrary waveform generator card?
A while ago, I tinkered a bit with running an awg on a raspberry pi pico. I ported the python code of this project to Rust, which lead to an even more impressive performance than described in the instructables article (notably, much faster load times). It uses the PIO and DMA for the signal generation, leaving the CPU basically idle after setup, the main restriction is memory for the waveform data. This generally presents quite a capable, small AWG.
I guess an RP2040, small output stage and single connector (using mini BNC? A regular one is probably too big) might be able to fit into a module.
What do you think?