Wow, really like the idea...
SG, you may want to update the OP, adding the fact that marks depend on skill trees and that stuff.
I agree that we are elementals of whatever element we join, since we start with that mark, but we can learn to be elementals from other elements (why not?)... That's how the skill trees work. Obviously you would lose reputation/bonuses/etc., but you would be both a
elemental and a
elemental, i.e. And you'll choose what powers to use before a fight. So it fits.
Apart from that, you'll be always restricted to choose other elements' cards, skills and/or marks, since you have to go to that elements' cities to learn them, so if you are in war with one element, you'll be unable to learn anything from them (unless you use that
skill, cloak I think)
Any bonus, up to what I know, is yet to be decided, but I like the mechanics as they are right now