One thing I like about it is that it would allow for one House to have spies in another House. It's hard to get a warrior into the Black sect to act as a spy for the Blades. But if all the Houses can take all classes it would be much easier to slip someone in. Just a thought. You could also add some pre-History so that none of the Houses like each other. And, like now, whichever House the Emperor belongs to could be given all sorts of perks (to the chagrin of the other Houses). Think "drow society" but with more than just drow.

Alternately, there could be a new mini-cabal centered around some drow secret society. Maybe they work to subvert other races, maybe they're an advance scout group for an invasion from the Underdark, who knows. Could possibly tie to them to the pair that took over Velkyn Oloth. Very weak powers (if any) but would add some role-play opportunities. I haven't thought alot about it, but I sort of like the idea of mini-cabals. Like a normal cabal w/ regard to role-play requirement and difficulty of entry, but with very weak powers (if any) and no item to defend. Basically it would amount to a very slight game-play reward for putting up some additional RP, and would provide another medium for players to interact with other players. Maybe only the leader gets any powers - having a cabal channel alone is enough of an advantage.