You know, I don't recall for sure -- don't have a blue book at hand -- but it seems to me that the standard units are defined by region; only the intraunits are defined by interest.
Now, the idea of creating a new unit down here is a little problematic because, as the discussion has shown, there's interest from areas that cover a couple of different existing units.
Ok, I don't think it's a good idea to "kidnap" folks into a new unit from several existing units when using the geographic definition. On the other hand, starting a new intraunit brings on its own issues; perhaps someone else can work on this angle.
Note, though, that it is not at all necessary to live in a region in order to be a member of the unit defined by that region. (Back when we had NM as our main membership, I recall seeing a membership list: There were folks from all over creation listed as members there.)
Perhaps, then, it's perfectly fine to start a new, non-encroaching, geographically defined NM unit. Just state in its goals what the unit is up to, what it's purpose/goal is. I mean, if you've got a NM unit that digs on camping in Alaska, that's perfectly legit!
Lynn