I play with a group that loves variety, and the idea of a two class game has been about the only complaint about Carcosa (or as one of my players constantly complains a one and a half.) This was the primary reason I came up with the Changeling idea, but another simpler method would seem to simply allow other OD&D classes in to the game.
I'm not a fan of thieves being as tough as fighters however, so I was wondering how others would handle hit points for such characters. Halving the result (rounding up) for the dice type roll might work, but the averages would end up a little odd. Any other ideas?