The Warlord we have now is a war/earth Orc. What I'm thinking is that not all warlords would be landlubbers. Why not make warlord training War & (Earth OR Water)? Then there would be two versions of the warlord, each with a slightly different ability cards.
Also, I think that there's a problem in the location of the arena being on land. It gives land dwelling creatures an advantage. The water/war mage herself would have to be able to breathe, not just the creatures she summons. Plus, how would a merfolk enter the arena? They don't have legs. I suppose a merfolk could use a wheelchair, if Etheria were technologically advanced enough.
Not all witches live in coastal towns. Pretty sure that aspect of the theme doesn't work.
Other than just another version of the warlord, or a siren, I have no idea what a war/water mage would be called. Maybe a Mermaster?