I run a 5-class 5-box and am levelling up 3 more classes as well (leaving out rogue and warrior).

Whatever you end up picking, I'd reccomend that two of them be a mage and a warlock. Just for pure NON-combat reasons.

Warlock for the summoning stone, and mage for the portals.

Every time you need to do something on just one of your toons, drop a summon stone and summon them back once they're done. Cuts your travel time in half.

Mage generates food for the whole team and you never ever have to worry about getting back to northrend/outland etc. You can set your whole group's hearthstones to convenient inns along the way for questing purposes.

While I've tried other comps for fun, I always miss having a mage or warlock along whenever I don't have one.