When the multiplayer mode is revamped, will the existing multiplayer format remain part of the game?
The current one will live on in a somewhat changed form: people won’t compete “live”, but instead in challenges that are available 24h and can be entered at any time. Once entered, you just like now have a certain time to complete the challenge and get updates on how other people have done via a leaderboard. There will be more modes than this though, which still are being discussed.
a built car entered into a challenge on some track? 
speaking of, is it possible to keep all the custom racetracks in one place for everyone to download via the game itself, without the need of the process we have now?
Well, when AutomationHub is finished, there will probably be a section of that to dedicated to tracks.
Not via the game, but you can find a section with tracks, yes and then manually import them into the game documents at C:\User\Automation\Tracks
There will be a mode with randomly generated tracks like the BRC. Just think: BRC with normal scenario limitations and 20min of time to get the whole car done
And for people that want one place to download all custom tracks, they can upload those to AutomationHub and have them all-together in one spot.
i do know where the tracks go and how manual mode works, but still would be nice if it would be done by the game itself