Whats a good house rule to give the Rogue a chance at winning Dungeon?
I would agree that the Rogue is the weakest class, although I don't have hard data to back that up. The reasoning is simple: she needs the same treasure count as the cleric, but is weaker in combat. Their special power to pass secret doors is a very poor compensation, not least because it's unreliable, and is easily copied with a secret door card.

The easiest adjustment is to change the amount of treasure required to win, but I have little idea what an appropriate target would be. Instead, you might consider adjusting the Rogue's special power:


Have Rogues find secret doors all the time, without a dice roll.
If they pick up a secret door card, treat it as a +1 magic sword (which compensates for their combat weakness).


Personally, I find it more fun just not to use the Rogue. Four is a sensible number for a good game of Dungeon, and two clerics and two warriors offers good balance, simple rules (no special powers) and plenty of competition without too many players hunting in the same levels.