Elbox opening anything up? Forget it.
Coldfire as a CPU upgrade path hasn't proven to be very valuable so far.
Other than that, sure co-operation is always nice thing to have. Not sure if it requires any "consortiom" however. Perhaps some loose setting with common single forum, irc channels etc?
Regarding compatibility: There already are several competing different solutions, each with slightly different approach. This gives them different bugs and slightly different behaviour. One thing that could be provided would be some sort of "acid test" to try to reveal bugs in the implementations. Then it would be easier to pinpoint and fix these bugs. There should probably be multiple tests, at least: cpu, blitter, copper, audio, cia (timers).
I'd think the priority should be in fixing the OCS implementation first, before trying to introduce ECS or AGA, or future extensions.
