My favorite house rule of all time is Hubris.
The idea for Hubris came from recognizing that some of my players were fool hardy and enjoyed getting messed up in conflict while others derived more joy from avoiding danger than anything else. The risk seekers had a tendency to take actions that got the risk adverse into equal trouble. This didn't seem fair. Basically what I wanted was a mechanism to treat different players differently without doing so randomly.
At the same time, there was general disagreement about what sort of game players wanted. Some wanted to be plucky heroes overcoming adversity and others wanted to be natural paragons kicking ass and taking names. It's tough to simulate both of those without treating people differently. Hubris was the answer.
Hubris only works if point buy is in the game. Granted, you don't have to actually generate stats with point buy, but the table is needed to determine Hubris. After abilities are generated, players can acquires additional ability points (to be spent according to the point buy schedule) by taking on Hubris. In the alternative, players could reduce stats or "sell" points left over from point buy back to acquire positive Hubris. Every point buy point costs -2 Hubris (and selling a point is +2 Hubris). No character may buy or sell points to acquire Hubris outside of +10 to -10.
Hubris is used to assess luck. A Hubris check is d20 + 1/2 level + Hubris (most often a penalty). Any player or the GM may call for a Hubris check. The GM then determines if the situation best calls for an individual Hubris check (i.e. a specific player), all players, or a group check. A group check uses the average Hubris of the entire party with a single d20 roll. The typical DC of a Hubris check is 10 for a "likely" event, 15 for an "unlikely" event, and 20 for an "extremely unlikely" event. Examples follow:
- Likely. Up ahead, the forest opens and you reach a fordable part of the river.
- Unlikely. Up ahead, an old abandoned ferry clings moored to the river bank.
- Extremely unlikely. Up ahead, a fisherman has hidden a well maintained boat in the reeds
The Hubris system allows variation in statistics and abilities to be introduced without the unfairness that sometimes accompanies randomness. It also sends informative signals to the GM with regards to which players welcome risk (i.e. will trade power for bad luck) versus which players are risk adverse (i.e. prefer no hubris for good luck). A player that takes on -10 Hubris for a handful of ability points is strongly indicating that he welcomes the risk. This signal is useful to the GM, gives the player what he wants, and lets you treat players differently with impunity later on.
Hubris checks are modified by level bonus so that as characters increase in level they gain greater control of their fate. Abilities represent a greater portion of total power at low levels, making them a potent benefit. At later levels, abilities are a lesser portion of total power and so it is appropriate that the Hubris penalty diminishes over time.
Two more important additions:
1. If a GM calls for a hubris check the penalty for failure can be anything the GM wants including the absence of luck. If a player calls for a hubris check, the penalty for failure is bad luck.
2. You don’t make a check to see how lucky you get. You pick how lucky you want to be (i.e. likely, unlikely, extremely unlikely) and see if your can make it happen.