I want to propose a site that I think would be very beneficial to its community, but technically the topic is programming and there's no real reason why questions couldn't be asked on StackOverflow. However, I still feel like a specific site for it would work much better for various reasons.
There's at least one example of a 'sub' domain -- the Game Development SE. But are there any guidelines in general about this kind of thing?
There are no hard-and-fast guidelines that I know of.
Some of what seems to matter:
How niche is your topic?
If it's of general interest (like the oh-no-not-again proposals for a JavaScript/jQuery site), then it won't happen. If it's incredibly focused—particularly if there are questions that are only programming-related when they're within your specialty—then it's more likely.
How popular is your topic?
If there are only five people in the world that are interested in your subject area, your proposal will never make it into commitment, much less beta.
Is your topic tied to a single vendor/product?
There are cases where this has worked (Ubuntu and Sharepoint, as just two examples), but in general, the SE community isn't crazy about single-vendor proposals.
