I have found that oily fingers, or traces of coolant on the fingers can cause a membrane to crack prematurely, or make it very difficult to clean. Other membrane keypads have a quite thick, resiliant covering membrane, but take a fair poke to activate, plus the same oily fingers, and cleaning solutions tend to wash the lettering off the keys.
A molded character in the "keycap" might help preserve the identity of the key. But washability is important, and it would be great if the keys had a click to them that was similar to a regular keyboard.
I'm interested to hear what you've got up your sleeve.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)