Only class left to add is Warmage. Or Battlemage, whichever. And I think it could work:
We have classes for melee+cleric, melee+druid, melee+shaman (BST), melee+necro, and melee+chanter (bard). All that's left now is melee+wiz+mage, a pure DPS class that fights at short range but also has lower-level pets, nukes, maybe even some self-only teleports. Would be supremely fun to play. Sort of a nuking rogue type deal. Only issue I can think of is that Ranger already kinda fills this role. However there are ways to make this different. Another issue is that adding it at level 120 or whatever would make it a non-factor on TLPs where most people would want to play them and get the lower-level experience. Introducing them at 120 and instantly obsoleting the first 119 levels would be iffy. The last class introduced was Zerker and they get a lot of nothing pre-60.
So I guess what I'd do is retroactively make the class unlock on TLPs at whichever early expansion makes the most sense. Maybe TSS or DoN. Or if they're feeling froggy, have it available from the start on Classic and drop it right around the time of the next TLP.
Dunno, there's a lot they could do to make something like this awesome and add some life to the game.