The campaign mode is likely going to be a combination of race events against AI, just like all the other GT's and probably some license test like challenges thrown in to better groom players to race online and obey the regulations and become more sportsmanlike. That is my guess.
For those who haven't played iRacing, let me tell you that the single most important thing added to GT with this release is the Sportsmanship rating system. iRacing has this and this is one of the main reasons you can have high quality racing experiences with complete strangers and not have to join leagues or race with only a small group of friends. There is nothing special about the average iRacing driver v. the average GT driver. The main factor as to why you get bumper cars online in GT and people trying their best to drive clean in iRacing is because of the sportsmanship rating penalty of of driving like a dick. It has nothing to do with the accuracy of the physics, damage, etc.
Without a sportsmanship rating, there is no incentive to not always try to get first place no matter how bad you smash your car. Cause it is a video game and you will never have to buy a new car or accidentally break your ribs, so without consequence, people naturally gravitate toward high risk behavior. This results in a shitty racing experience. Sportsmanship is that consequence. Drive like a dick, get forced to race against other dicks or even locked out of the higher class races. If you drive a Hyundai like a dick, why let you join in against people racing GT1 cars for you to only fuck up things even worse?
Right now, the sportmanship penalties are a little loose. IMO, they need to be really severe because they have to simulate that real world thought of... oh shit I don't want to fuck my car up and pay thousands of dollars, or maybe I wont do this insane pass with a 1% chance of success cause I would hit that wall and maybe die. Once you get that virtual equivalent of caution, people of all talents begin to race smart and have really exciting and rewarding racing experiences. I never got very good at iRacing but I still had some of my most thrilling online races ever in the lower class races.
I hope GTS does the same driver rating points system too where depending on the quality of people you are racing against, the more your efforts are rewarded. So if I am a D class racer and there happened to be a few A and B racers in my race, if I get 4th place in that race, it is worth more ranking points than if I got first place against a bunch of E class racers. This adds that final real life scenario of playing it smart when you know 3rd place is a good result and not going full kamikaze just to get first at all costs.
Assuming they resolve the technical side of the online races, the sportsmanship system is what is likely going to make or break this game for setting a new benchmark in quality online racing on consoles.