You have my tentative attention. My first thought was a planeswalker from a homebrew plane (I've been working on a similar idea for a while), but then I realised that I wouldn't quite be able to translate that into spells that have already been seen as far as we know.

So now I'm thinking - haven't decided if I'm properly interested - along the lines of a Zendikar-born 'walker who has strong connections to the Expeditionary Houses (meaning their magic would focus on Allies in particular). First thoughts are white, green, and either red or blue.