I'm about halfway through developing an elemental guard prestige class, which will sort of be like OA elemental guard meets swiftblade. It should make a good gishy class, with powerful, crunchy abilities, although you won't get 9th level spells if you take all 10 levels of it.

I doubt I'd make a prestige class to further specialize the elemental casters into their respective elements; feats and ACFs would be more appropriate for that.

What seems iffy about the master of elements?