I think something like a touch-based display would be cool.

Like an array of low cost transducers (maybe there's a way to multiplex the output mechanically like we do with electronics (so only 2*sqrt(N) motors are needed), but in mechanics inertia and speed should be a more significant problem)

I 6x6 matrix of solenoids for example doesn't cost much, and you could reasonably control them individually I think?