Proposals are meant to create new subjects not already covered by our existing sites. We do not generally split off sites simply to give a topic its own space.
In very rare circumstance where we might consider spinning off a site, the bar is a quite high since (presumably) the subject is already being served by live Stack Exchange site. For example, we would need a clear demonstration that such a site serves a new audience not currently reached by our network. We may ask an approaching audience to help expand the reach of one our existing sites, but we do not generally create new sites simply to split off or gather up subjects already covered elsewhere. We would be looking for things like:
- An substantial ecosystem of questions that are not already in scope on existing site(s)
- A show of numbers that clearly demonstrates the parent site is failing to attract that audience
- A weighty argument that a new community exists that does not identify themselves as part of the larger subject (like Ubuntu versus Unix)
- Scope Failure — Maybe we got it wrong the first time. A large subject can cover a huge number of topics. But if the site is not competitive in any particular subject, it may be time for a breakup ("sum of the parts")
- The "Act of God" Scenario — If Elon Musk came to us personally with the support to become The Hub for electric vehicle questions, we might make that happen.
(I'll add to this list as other "splitting" scenarios emerge, but there are many other criteria that go into the evaluation of a proposal)