One issue that has been noticed in online games is that some more practiced and skilled players (who I will call 'expert' players) are able to complete tracks much more quickly than regular players.
There has been some discussion of how to help balance online games, which you can find in this thread. One of the main issues is figuring out how the game can be unfair to expert players without making them feel cheated or bored.
One idea I've proposed is using temporal or dynamic obstacles in the maps which will affect expert players more than other players. I think this could be a very useful technique, if well implemented. Discussing this among the STK community could result in interesting ideas to help liven up maps.
Basic examples of moving obstacles already exist, such as the large cannonball in Fort Magma and the train in Black Forest. These hazards can be coordinated so they occur more often earlier in the game or lap and occur less later. This means that an expert player who gets there early has more hazards to dodge that may slow them down, while a slower player has less to avoid.
One example is a place in the track with falling rocks, where more rocks fall earlier in the lap. Like the cannonball in Fort Magma, the rocks would bounce on the track before leaving so that later players won't be affected. The rocks could be replaced with something else depending on the track theme, maybe stalactites, golf balls or car traffic.
A more complicated example would be creating a shortcut that expert player are discouraged from using, that will allow less skilled players to catch up. I have drawn up a simple track map that demonstrates what this could look like.
In this track, there is a shortcut that is completely blocked by a slow train for a while, forcing expert players who arrive early to take a more challenging route. Once the train has finally passed, players can now take a shorter route that will reduce the gap between them and the expert player/s.
I have also added a second train at the end of the shortcut. This means that an unsporting expert player who simply stops and waits for the train to move should not beat an expert player who immediately takes the long route. An expert player is disadvantaged by taking the shortcut, but slower players are rewarded, as shown by the small table in the image. If designed well, this kind of alt route could help force more balanced games even if the players have very different skill, without boring the expert player.
Challenges:
These are not easy, fool-proof solutions. Some potential issues I can think of are:
1) a hazard shouldn't discourage good players to drive well, but it should challenge them more. This may a hard balance to find. Why shouldn't they just stop and wait for the hazard to pass and then drive well?
2) if a track layout allows an expert player to overlap a slower amateur player, a very difficult hazard built to challenge the expert player every lap could affect the slow player! That would be especially mean and insulting.
3) timing and placement of hazards is critical so that expert players are penalized more than others. Badly-timed hazards would be worse than no hazards. Maybe triggers on the track could be used for timing when hazards begin and end.
4) An expert player could memorize a safe route through difficult hazards, or the timing of when a hazard stops. Randomization through scripting would prevent this to keep it challenging.