Anyway, you should probably have something more to distinguish yourself besides just being online-only. The Volity system is actually pretty interesting; it works over Jabber.
There are a ton of sites in the multi-player gaming niche. I think there's still lots of room for competition, and many ways to compete with existing sites. Whether on game selection / implementation or technology improvements.
They can still use Java and let users launch the application via Java Web Start.