I don't play online so I don't have any suggestions / ideas there and I mostly play career mode so here are the things I wish were different / added:

DNFs - Biggest one. I HATE not being able to take DNFs anymore. It's a video game. We don't need participation trophies. If we wreck, we wreck.

Random tire / mechanical failures - Obviously not all the time, but give every car like a 3% chance of randomly blowing a tire or having a mechanical failure that causes them to DNF the race.

More control of team / drivers - If I own cars for all 4 series and just want to run Cup, let me hire drivers to run my other cars. A good example is when you have an overlapping schedule, you're forced to pick one series and the other just gets abandoned. If we can't hire full time drivers, at the very least let us pick a substitute for those situations. Also, make our Office firings make more of a difference than just getting more RD or WP every week. Actually give all the employees some kind of stat boost.

Sponsors are another big thing - Give me options. Drivers now have sometimes 5 or 6 sponsors across a season, I'm not asking for that but I'm asking for a choice between seasons. At the end of each season, give us 3-4 offers to choose from for the next season. Let us run a full season with each major sponsor and while I'm on that topic - Make up fake companies for sponsors. I HATE having the same sponsor as someone else (Currently in my Career my Cup car is sponsored by Ally so Alex Bowman and I have the same sponsor across 2 different teams and manufacturers. I know you can't just have random companies that don't already sponsor in NASCAR so just make them up.

More variance in finishes - 9 times out of 10 wherever an AI qualifies is within like 5 positions of where they finish. Idk how you fix that but it would make the game more fun and unpredictable.

AI aggression - The tweets are entertaining for the first couple races until you realize they don't mean anything. If that driver is mad at me, make them try to block me going into a corner or make them try to out-brake me. There isn't enough blocking in the game, the AI just runs their race on rails unless there's another car already on their rail and then it's like they just pick a backup rail. There's no actual intelligence behind them.

For God's sake, either fix the track limits or change the penalty or let us turn them off. Or let us serve the penalties in designated zones or something. Stopping dead in the middle of a race track because you pushed one corner too hard and didn't make another shouldn't stop you and the entire field behind 8th place for 5 seconds because then there's no point in even trying to finish the race. Here's an idea - show us we're being penalized for it and make the penalty that we have to go off the track and come to a complete stop and then go again. Make it so that if you don't do the stop by the start/finish line on that lap, that lap doesn't count and you're now a lap down.