Probably a ridiculous and technically impossible idea, got it while watching some races I was not able to attend.

The idea is to allow the player to leave the pits during replay viewing, so that one could get a taste of the race, react to situations which are not easy to reproduce online, race head to head with your early-career selves. User's car couldn't interact with the cars in the replay, but perhaps could get their tow, be exposed to the current wind conditions, etc.

Error will happen till contact comes or changes physical effects, slipstream for example.

In some other sims it's possible to join multiplayer race as a ghost car. Could the implementation be something similar?

Not essentially, we're talking replays here. Of course it would have its limitations, as you could plough through everyone without a care in the world, but used with a bit of imagination it could be a fantastic training tool.

I'm also not convinced about the errors UM21 mentioned, as it's entirely possible to make someone's car invisible to collision physics.

I see what you mean, but... isn't that a bit... hm... hmmm... I see no real benefit though.

I had a long response written out but it didn't go through, now I am annoyed and will be short and to the point.

This is useful because you can chase a ghost replay and adjust your driving to try and beat it. It's a visual representation of a time based on a saved replay. You can't collide or affect the ghost in any way, it is just a replay on its own track, your opponent is still the clock just with a fancy marker to battle.

Check out Forza 4 or one of the others in the series, their Hot Lap mode has ghosts which are really useful for tweaking/adjusting your technique

I think this would be great, it should 100% be implemented, just a replay hotlap mode

so obviously this clip is a little silly but you can see the ghost in it, he's drifting trying to ride in tandem with the ghost

it's just an earlier lap that it saved that he can now drive with, in Forza there could be multiple instances of ghosts on a lap and you can battle with 3 or 4 at a time, it would even automatically add your fastest ghosts replacing the old ones so you're constantly pushed to go faster and faster and keep pushing your time down. It's a good system, would love to see client support but perhaps one of the brilliant modders/addon makers could develop a workaround for this? It would be incredible!

https://www.youtube.com/watch?v=sXICRr-x8b0

Quote from BaronX :

I think this would be great, it should 100% be implemented, just a replay hotlap mode

So you are talking about singleplayer replay joining possibility. For example in Raceroom you have a hotlap-kind mode (Leaderboard mode) where you can select one result from the board and have it as a ghost car while you run your hotlap.

Yes, that is what I mean, I don't know if that is what OP had in mind but I think it would be terrific to have such functionality. Perhaps OP could get what he wanted in being able to join a race from the past through a replay, though perhaps his suggestion was to be able to join a regular multiplayer lobby AS a ghost so you can't collide or cause problems, I wouldn't mind that feature personally so I can race hard but not crash with the fast guys while practicing my own lines/technique.