Pally Mage seens like a good combo. Mages are glass cannons, so having a aoe tank keeping the mobs busy, u can fire away with the mage and not worry as much about him dying. Plus u get ports/water, and a nice INT buff, which helps AoE paladins lack of mana. Then ud have 2 of the best "boosting" classes, should u ever decide to go more than 2.

Other than that, any slave caster will do a fine job backing up an AoE Paladin, and any other AoE class will have good synergy. If you keep with the lock, u can use hellfire, and keep SS on the pally.

If you add a healer to tail the paladin, priest seems like a good match there, they give stam buff, excellent healing, and youd be a godsend for many people as you can tank/heal, and everyone else can just dps. Should make instancing pretty easy, especially later in the game, and heroics.