I like the challenge of trying different technologies, although my team members prefer simplicity and robustness.
In theory a continuously variable transmission is the way to go, sensing the motor speed and adapt the gear ratio to it.
You'll find already so many different ideas and patents out there and you may even invent something new.
After some web-search, I found this one, which is my personal favor:
http://www.fallbrooktech.com/NuVinci.asp