What we've got so far!
- Simbin penalty rules: i.e. a drive through ... .
Damage: damage won't be a solution. AFAIK, damage is only cosmetic. There is no mechanical/electronic damage, so a driver is still capable of driving competitive with a damaged car. And AFAIK, only race cars have damage. So if you participate in an online race with only stock cars, and these cars don't have damage, in that case we need a penalty system.
License:
... I hope there is some kind of license system in future racing games. You need to earn your advanced license from driving carefully. And you might lose your advance license if you start to drive carelessly. Player games can be set with different licence requirements so you could choose what kind of players you would like to play with.
Flag rules: I just came up with this.

The black flag
The solid black flag is used to summon a driver to the pits. It is usually used to punish a driver or team for disobeying the rules, but may also be used when a car is suffering a dangerous mechanical failure, such as a loose hood or dragging bumper, or even calling a driver to the pits when their radio is not working. The car number of the summoned driver is displayed in a designated place near the flagstand or occasionally on the flag itself. Black flags can be waved at all observation posts simultaneously to order all drivers to clear the track after the starter waves the red flag, often in the case of a serious accident.
Black flag with orange circle
A black flag with an orange disk in its center indicates that a car is being summoned to the pits due to mechanical problems that are interfering with the race, such as an oil, water, or fuel leak.
The per-bend black/white flag
A diagonally-divided black-and-white flag is displayed with a car number to indicate a penalty for
unsportsmanlike conduct. This flag can be displayed if a car tries to intentionally drive another car off the course, or if a driver gets out of his/her car and initiates an altercation with another driver.
Other administrators do not distinguish mechanical problems or unsportsmanlike conduct from rules violations.
The white cross flag
Some leagues use a black flag with a white
saltire. It is displayed with a car number if a driver ignores the other black flags for an extended period of time, and indicates that that car is no longer being scored. In NASCAR as well as the IRL, once this flag is displayed, the car is not scored again until it answers the black flag by pitting.
EDIT: if a driver doesn't comply, the game (GT5's AI) will take over and drives his/her car into the pit.