I think the 15 nobles threshold to found a new realm is reasonable enough. The game doesn't really need new realms at this stage, anyway. The conversion of Alara to Sydgard shows there are other ways to have fresh starts and try out new things. The same goes for realm mergers. Merge realms and create something new out of the merger (or meld existing cultures to provide a new twist).
New realms founded with a few nobles tend to be vanity projects that some times hit gold, but more often crash and burn.
