For the sake of something different and a bit of fun, what about a Lego world? Surely there would be co-marketing opportunities with that to help finance it and the appeal of Lego is pretty broad.
This could also provide an opportunity to introduce a route builder tool so that people can experiment with their favourite sections of road. Even if it’s just a ‘mini game’ within Zwift it could reveal a host of new route options that could eventually become standard for everyone to choose. Zwift devs could just watch what people build for inspiration!
Oh, and while you’re at it, let people share their routes. That could really add to the community feel of Zwift.