Since your main is a paladin, you can experience tanking, healing as well as melee dps, though as a ret paly, its really hard to find a 5 man group. So I believe your first alt should roll a class that can play as a caster: mage, warlock, druid, shaman, priest, or a hunter which is a ranged dps as well.
But your gf has a mage, i guess you can experience caster dps by playing her character. ;o If so, I say hunter is your best choice, if you want to have some orginal melee dps experience, roll rogue, easier to find a 5 man group compare to dps warrior.
If you want to experience wow as much as you can within 2 characters, roll druid, or shaman, all hyprid class.
For myself, I have mage, lock, paladin, shaman, priest, and a on the way to 70 rogue.

have lv60 warrior, hunter and druid too, but on other server/account.
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