This is something I'd like to do for 1.0, and also one of the reasons that I think the 1.0 name is useful. It's a non-backwards compatible change, but one that's small and useful.
I'm leaning towards .ade for the parallel with Processing and .pde, but .ino is tempting too.
Adding a configuration switch in the preferences would be handy to control this behaviour in case backwards compatibility is needed.