Well, there are a few combos which would make for good additions to the possible amount of combos right now. A few include:
Gnome Hunter - There is no reason why gnomes should not be hunters. Anyone who would raise the "issue" of them being too technologically focused and/or detached from nature should take a look at goblins who can be hunters.
Human Shaman - Before anyone tries to argue this, they have already been seen in plentiful numbers within the Twilight's Hammer, not to mention there are several independent individuals such as Highlord Demitrian. To add an incentive, adding this as a choice would boost the number of shaman players in the Alliance by a truckload.
Goblin & Worgen Monks should be included after MoP as well. They weren't included for what I believe surrounded some concern pertaining to population balance. But after MoP if Blizzard allowed for goblins and worgen to become monks, we won't have to worry nearly as much about too many monks being either of these two races.
Unfortunately I am having difficulty with adding Horde race/class combos, any ideas provided with good support I will add to the list. I thought about Orc Priests or Blood Elf Druids at the most, but I am sure that we can understand why those ideas might be difficult to pass by.
Maybe the Goblins could begin making "transactions" with nature...