Poured a lot of my time into this back when I still had any, my crowning achievement still being the fabled "round-the-world" trip, solely with propeller planes. Learned a lot about the places that I made stops in.
The franchise did something spectacularly well that any RPG publishers might do well to take note of: It embraced modding without any restrictions. Anyone was (is?) free to create and distribute 3rd-party add-ons, including commercially. This has led to literally hundreds of thousands of add-ons, from texture mods over new airplane models to entire countries, many of them of exceptional quality. I am sure that this did a lot to extend the shelf-life of the franchise, just as add-ons did for Morrowind - but with permissive monetization options that allowed independent vendors to really raise their level of quality without being funnelled into a specific storefront or "workshop" or "Creation Club" or other such silly business.