Positional geomancing barrier erecting mage? Say you can cast barriers that will soak damage when you stand behind them, barriers significantly reduce the damage when you're the target (can soak for other players but not as well it would for you as mage tank). Allow the mage a few mobility spells (teleport to location, teleport to geo node) so they can get around and set up zones and (trip wire, aoe hate zone, stun box, wall boons ect). To make moving around not ruin positional jobs I suppose you could give him a plane shift sort of spell, a physical copy of the tank and target monster are locked in battle/position at 50% reduced damage ability while aether copies are not locked the aether copies deal 30% of original damage. The point here is that you could drag the boss around, toss up a secondary barrier to protect some mages against a sly slipstream (30% damage much easier to negate), drop down a geo node, and then at the end of the spell get snapped back to your position (health synced between the copies). It might be a bit too hard to balance around certain boss mechanics but I kind of think it would be nice to have a mage that felt like a "mage" but also tanked (you're not going to eat the attacks up front, you'll rely on your spells).
Could go crazy and make it a sort of geo engineer (walls can topple over or turn into giant shrapnel grenades as DD options at the cost of defense, certain nodes can turn into turrets). There'd have to be a good system that maxed nodes or certain actions though, I can imagine a big mess on screen if you could have a bunch of nodes up and everyone in party was this weird geo engineer tank. (Can make it a dual job so there was a positional engineer tank and positional engineer DD/Healer)