Of course improving weather is a good option. Though I'd prefer to see a new implementation of it than the slots. I'd much rather have a % of rain for the race and the game decides the weather. Maybe your engineer can tell you what the percentage chance is as well.
For example: "The chance of rain is 70% in the next 30 minutes. But there is no rain expected for the rest of the race beyond that."
"The chance of rain for this hour is 25% but it is not expected to arrive for another 50 minutes."
I love this. It's very similar to how Gran Turismo 5/6 handled weather but with the engineer jumping in to help you. If it starts raining in a location before you see it, it might help if the engineer notified you before you approached that wet section. "It's raining in Sector 2. It's raining between Indianapolis and Mulsanne."
I am a big online endurance racing fan, so all my suggestions will reflect that.
Online Driver Swaps: Easily at the top of my list of features I want to see. You can take it further if you allowed to take your "career mode" online in a sense that you and your friends race a car against the AI.
Driver Rating: It's very easy to move up in safety rank and hard to move down. Thus, you can still run into dirty drivers that are ranked B, A, and S that will do anything such that their skill rating remains high.
The skill rating is too focused on how you finish relative to the rank of the drivers around you. It should be more nuanced than this. There needs to be an element of how quickly you can drive the car. If you are taken out by a dirty driver in turn 1, you are likely going to lose many points that race. However, if that driver recovered many positions after being taken out or had a string of very fast laps to catch up, the driver is still indeed skilled relative to their competition. Since some classes are imbalanced on purpose to mimic real-life performance, how well you perform or finish in a slower or faster car demonstrates your skill also. Winning a race in the Mercedes CLK is far less impressive than winning in the Nissan R390. Your skill rating would not properly reflect that in its current state. People avoid some of these classes because only 1 or 2 of 6+ cars are capable of winning on any given track. If people are rewarded for challenging themselves and trying slower cars, we'd see less "forced" one-make races and a smaller majority of GT3/GTE rooms.
The driver rating I believe should be able to be simplified into Gold, Silver, Platinum, etc. It'd be nice to have races with driver swaps with different driver ratings to simulate a PRO-AM lineup. In reference to the last paragraph, some classes could be more interesting of "PRO" drivers weren't able to pick the fastest cars in an imbalanced class, giving slower drivers a chance in a faster car.
Matchmaking: This in my opinion is why the driver rating feature didn't go as far as it could. Without a tool to group link minded racers, you lose the significance of the driver rating and its exigence.
Tire Compounds: At the most, we get soft, hard, and wet compounds on a race car in the game. However, we're given the widest range and fidelity of weather and temperature conditions than any other sim ever made. I'm ignoring all-weather and ice tires because they wouldn't get used in real life. I think part of the reason why people hate how unforgiving puddles are in the game is because we seem to not have dedicated full wet tires in the game. The wet tires in this game seem to be a jack of all trades, master of none. If we had intermediates, wets, and full wets, we might have an easier time managing all the different rain conditions. Some classes in real life do not have a range of wet tires to choose from, but in the classes this exists, we should have that option. This also goes for medium tires in the dry because some soft and hard tires have no overlap for what conditions they like to be used in.
Rules: If all of the above were to happen, I'd like to see more options in the rules when setting up lobbies. These can be simple such as 2 mandatory pit stops or mandatory tire change. It could get more detailed such as a minimum driver stint time or how much fuel you're allowed to use in a race (see Group C). DRS could use some sort of rule rather than a free for all. It can be as simple as having DRS for an entire sector if you're within a second of the car ahead at the end of the previous sector. This way, you don't have to program dedicated DRS zones even though that would be the most realistic alternative to what we currently have.
Fictional Open Wheelers: Minor suggestion, but I'd love to see multiple variations of the Formula A/X to replicate different chassis, aero, power units etc. These classes don't need to be spec. Having two chassis options and two power units out of these cars would give us four total combinations. Furthermore, it'd be nice to see different Formula As from different eras. A 90s and 00s (with tire war?) Formula A would be a lot of fun.
Driveclub has great road based tracks. I would love to drive them in more simulation physics 
