I Personally think a "pokedex" of all the pokemon in avatar form would be the best approach.
The drop-down list could have every pokemon in alphabetical or numberical order, and each pokemon could have several avatars.
I Think that this would allow users old and new a bigger selection with the ability to choose an avatar that represents them best. thats what avatars are about!
as for the anime characters, its not a bad idea, but this is a pokemon forum, so i dont think they'd get used as much, since mainly newcomers use the onboard avatars, and more experienced pc users get our skilled artists to make them one- usually with a matching sig.
However, if there is demand for it, then you might have caught on to a good idea?