If you require a lot of people with lots of reputation across the SO sites, doesn't that mean only programmers will be attracted? In order to get a proposal to beta, you need high rep users, while those are not always experts on the site you are creating.
Sure, you can try to attract lots of 1 rep new users from across the internet, but they will never match up with some high rep users that have been around a bit. So if you're trying to create a site that has nothing at all to do with computers (for that's what most people here are interested in), you can probably never get it past commitment phase. The current system will only spawn sites that programmers have at least some interest in, or take incredibly long for other sites.