I should also point out that I really don't think you can get success by explicitly starting out trying to replace these existing communities. Rather you should

I should also point out that I really don't think you can get success by explicitly starting out trying to replace these existing communities. Rather you should figure out how you can compliment them and provide what isn't there or address existing concerns. For example, you can try aggregating content if the communities are spread out and serve as a sort of "hub"