I've talked about Skaven before and Planetside.
So my quick two cents.
- Grey Seer - Equivalent to Archmage and Shaman. Variously imbibing Warpstone and expelling it to weave spells.
- Plague Monk - Equivalent to Rune Priest and Zealot (this one is Sareinis)
- Warpfire Cannon Skaven - Equivalent to Engineer or Magus. Guns ho!
- Assassain - Witch Elves and Witch Hunters suddenly find sneaky rats butting in.
- Rat Ogre Tamer - White Lions and Squig Herders.
But one race alone wouldn't properly shake up RvR, it'd have to be an entire faction. Unless you go the Planetside Black Ops route.
If Skaven do make an appearance and exist primarily to foul up the plans of everyone else...well that's Skaven-y and perfectly fine by me.