for RPG purposes I'd say that anyone (or at least any potential bender) can initially learn any element, but apart from the Avatar you're locked into that element.

Pretty much. According to the creators (and one or two characters in the show), the difference between benders and non-benders tends to be at least partly genetic, but the difference between, say, a Waterbender and a Firebender is much more sociopolitical/religious.