I think the ability to upgrade small and medium size runways to larger ones would be a good option like how you and approach lights. Click the upgrade button then choose the next size up. You can limit it to the next size or even allow upgrade from small to large in one step of wanted but can see the reasoning behind requiring small-medium-large and not small-large.
Why it should be implemented:
With the limited map space you do not always have the real estate to plop down another runway and closing, destroying, and building a larger runway takes alot of in-game time and financial hit on your airport do to the lack of flights coming in
How would it work though, would it arbitrarily upgrade to the left or right-hand side? Deleting a runway gives you 50% cash back I think, this just sounds like a nightmare-implementation feature unfortunately as every runway has a different length.
Best bet is to build a temporary runway the best you can, and if you’re too tight on space, well many airports in real life suffer from the same and they just have to shut down some flights (aprons in ACEO, you can disable auto-planner on them, cancel the next flights and when it’s empty just turn it off).
With a loan you should be able to get your profit margin back. I’m operating my airport on just 13 small aircraft aprons + ~7 general aviation and make very good profits (runway landing price to 1,000 and everything else in the limit of the green).
@ThisIsTheJohnDoe, this feature was already implemented. So, how it would work is already in code. However, the feature was removed in later alpha versions.
