Could we add a custom close reason for questions in proposal mode to close a question as:
Already on topic at current site: (include link to current SE)
I think this reason would help us with forming sites that minimise the overlap with existing SE sites.
This unravels a general discoverability problem inherent to what Area 51 is about. On the one hand, you want your example questions to show there's enough unique content to justify the creation of a new site… yet you don't want these (hypothetical) cross-site closures to become an all-out moratorium against every question which just happens to be on topic on another site.
It's not an unsolvable problem, but the close reasons were derived from the original Stack Exchange system, so changing them piecemeal will likely introduce undesirable behaviors without further UI development and support. It's not yet clear whether any future development in Area 51 would be geared towards fixing Area 51 or developing a whole new system ("Stack Exchange Labs"). My vote is for the latter — unfortunately, I do not have an update on when or if that will ever occur.
For the time being, this is probably better handled through comments and voting for the best example questions — the question which make the best possible case for building that site.
I am going to leave this un-status'd since it is neither declined nor implementable at this time.
