You’re saying you don’t need the money but you want the experience?
It sounds like you may already have gotten an MEng or the equivalent, but if you haven't, an MEng is a way to work on a cutting edge project.
One goal is to develop a really good demo, as opposed to a company. Personally I had a lot of success making demos when I was working full time for somebody else but when I was doing it full-time on my own account (as well as doing a lot of business development work) I didn't quite find what I was looking for.