In the current state of the game, the levels are not very representative of all the cool things airplanes can do (Other than blow stuff up and go fast), which I think is important for a game about airplanes.

Aerial Firefighting
There could be levels with fires in varying locations on the islands with different levels of difficulty. For example, a fire in the canyons around Krakabloa would be harder to extinguish than one on the North Wright Runway island. And of course, a larger fire is harder to extinguish than a smaller one.

There could also be a challenge like the bombing challenge to see how many small fires you can extinguish as they pop up around Wright island in a certain amount of time, with each fire extinguished giving a time bonus.

Although, this would require the devs adding a water or fire retardant dispensing block. And I'm not sure how hard it would be to implement a limited supply of it.

Landing Challenges
Similar to MFS 2020, there could be landing challenges with various wind and time conditions or even aircraft requirements (i.e. minimum weight) or starting location to increase the difficulty. Of course, you can do these on your own, but to add a sense of accomplishment, a score system could be added based on descent rate, rolling time, where you touch down, closeness to the center line, etc.

Bush Flying Challenges
This would be simply takeoff and landing in remote locations across the Wright Isles. It does feel a little "Just fly from point A to point B," but there is difficulty in landing and taking off from rough terrain, I suppose.

Like the firefighting challenge idea, there could also be a mode where you have to land at designated spots on Wright Island and gain time for each landing.

Cargo Transport
We literally have magnets...
Seriously though, it would be interesting to transport some shipping containers or other things. And to make the levels more difficult, the weight of the cargo can vary, requiring heavier duty aircraft.

Although, unless you have something like a Skycrane, there isn't a really intuitive way of loading cargo onto aircraft in the game yet. So maybe there could be a cargo block that looks like a shipping container that would need to be added to the aircraft. Of course, to be valid for the level, it's XML values would not be allowed to be changed.

More Fleshed Out Combat Levels
We already have some combat levels in-game, but more could be added. There is a WW2 dogfight, but no modern dogfight, a WW2 destroyer assault, but no modern one, etc. I'm sure there are other new levels that could be added as well, perhaps adding the missions from the Combat Challenges Mod into the base game?

Plus, we need more SimpleLore.

I'm very certain this custodian will become a martyr for his rebel spy friends. And what the hell is this ultimate weapon we're protecting?

Those are just my thoughts.

Of course, you could just cheese ALL of these with XML magic, but you're not going to. And we both know it.