So, I hate the flat-surfaced keys you find for the two-part keycaps where you can remove the top part and insert your own legend, so I wanted to run an idea past people who create keycaps which could probably be a commercial thing:
Create a standard MX-compatible button base (about half the height of a normal key) with a flat or curved plate on the top for users to then insert their legend, and a transparent top part which fits on top of the base, but which has a curved surface. Getting the two together to be a tight fit may be an issue, but I've never seen blank curved keycaps like this.
Remember me when you make a million!