I'm a bit of a map nut, and have already experimented with this. Truth be told, the current layout (4x3) fits comfortably on a standard folding table, with just enough room for all the on-board craziness that occurs.
While I like where you're going with this, i have a few observations.
The 4x4 grid may seem like a great option for multiplayer as it adds a little more breather room, but it goes against the whole reason the game is demonstrated on 3x2 and played on a 4x3 board: There's no running away in mage wars! Ranges cap out at 2 in most cases, meaning that a 4x4 board would only encourage turtle strategies. While those can be interesting (The Warlord jumps immediately to mind!) the whole point of the small arena size is to force player interaction early and often.
Hexes + Mage Wars = Disaster! I've personally experimented with a 4x3 hex layout, and it doesn't seem the rules tolerate that much. One of the big concepts of the game is quadratic (4-direction) movement and adjacancy, so adding more just makes some spells insane, like Fireball, and some abilities almost useless like Outposts.
What i'd like to suggest is changing the arena by adding forced walls. Toss in a few static Stone Walls at the start of the game at random, and see what happens.