Apologes if it doesn't make sense or isn't feasible but the general idea would be to support the ability to play multiplayer online as you do today, but also go back to a saved shared session you were playing with a friend at any time, even if either or both of you have played online since.
At the moment, a stopped game will be retained, but I noticed that unless you both keep that session, you will likely lose your truck positions etc. If one of you keeps the session, the other can rejoin, but it doesn't seem to always retain the other players' data.
For example I was playing with a friend for an evening and then we finished up. I went on to play MP online, therefore losing that session. When we rejoined a few days later on his saved session, my progress looked to have been reset such that I had to unlock my vehicles all over again.
It would be great if I was able to save a few different ongoing sessions so that I could return to them at any point (or when that friend was online again), whilst still playing online in my current 'autosave' session.
Hope that makes some kind of sense