I have been playing for a week now and having come from some very popular games with large followings, I feel like some of the suggestions below could truly bring some new community and engagement to the game.
1. General leagues:
I think a downside to the game is starting off unaware many aspects of the game while trying to track down help. This is 100% normal as it's exciting to explore, but joining a league which has mostly inactive players or does not seem to have much going on deters the player from sticking around to figure the game out.
I think there should be a general rookie, pro, and elite championship that you work through when you enter. I think requiring the user to enter a league immediately ends with them possibly finding a dead league or being confused and just giving up on the game. Because end of the day, it's the community that keeps users around.
OOTP Perfect team, for example, has a tourney system based on promotion/relegation amongst a large user base so I believe this is possible for iGP.
2. Have separate weekly/monthly competitions with unique prizes or liveries. Having something that everybody strategizes around can create community by discussing game plans, pit stops, etc.
I know you cannot have 100 people racing so maybe taking fastest times and then have tieed prizes at that point. So people can continue to race and improve their time and track the leaderboards throughout the competition.
But for this competition, everything must be equal so that newer players can also compete. You could assign a set number of design points, driver energy, etc. and you must find the perfect mix for the best times.
3. Make offline only leagues more known. A lot of people are busy and can't commit to games like they used to, hence why idle games are so popular now. So make a game mode more easily visible which allows in game strategy to be bypassed and more focused on race prep and upgrades.
4. Official discord and community manager
Most people immediately join discord to see the community engagement, I think having announcements and more communication via discord could be beneficial. This is also a hub where similar to general leagues, you initially go and either stay or then at that point find specific leagues.
An example to flesh the game out more is to have livery competitions. Allow players to submit designs and the community votes. This starts to create passion for the game outside of actual gameplay. If compatibility isn't an issue, possibly even use the winners livery as a reward in game. People love to see that user content can be found in game.
These just came to mind and I hope I do not come across arrogant, I just think this game has amazing depth and talented team behind it and thought I could offer a few ideas to help it get the full recognition it deserves. I am also aware I am new to the game and could be off base on some of my assumptions. Please comment and let me know!
See you on the track!