My ideas as a game developer. (not the best a coding but I enjoy game design)
The Base for this game is already amazing so more similar content would only make it better so I hope you are still working on it.
-Name the current mode infinite mode
-Have multiple modes.
-Have multiple difficulties (specifically for infinite mode). My ideas are Learner, Novice and Pro.
Learner would have landmarks on the map to help orient themselves and get and understanding on where they are, they would also have an arrow appear for a short amount of time when they collect a package pointing to where the country is, and they would also have the option after a time to abandon their package in hopes of getting an easier one an easier one, they would also have access to the map but the map would not have names on.
The Novice would have the same features as easy mode excluding the pointing arrow and the land marks.
The Pro would have none of those features.
This would mean that even if the player is new to geography they can teach themselves without being told what to do but having things to help.
-Ideas for modes. Package Plus, Flag Finder and Flying Frenzy.
In package plus your employers have payed for a package plus subscription so they can get their package faster. You are given 3 packages to collect and deliver and are given a score based on distance from point and time.
In Flag finder, the rules would be the same as infinite mode on pro difficulty, however, all of the countries have flags covering them instead of normal terrain and you have to find and deliver to the country based on its flag.
In Flying Frenzy, all of the planes are in a rush. This mode is also the same as infinite mode but there is other planes flaying around at various speeds that you must dodge while delivering packages.
-Cosmetics. From my experience playing games in this style, I think unlockable cosmetics make people stop playing. People unlock all the cosmetics and think they have done everything and have no reason to play. So, I think cosmetics should be unlocked from the start but there are special ones that are achieved through milestones. Similar to the gold weapon skins for headshots in call of duty. There could be many of these as there are lots of things that could be customised. (Music, plane trail, plain pattern, pattern colours, Hud maybe, boost trail, boost sound, plane sound, package looks)
Also players in infinite mode (and maybe others) should be able to choose between always day, always night, half day/half night, or time changing as you play.
Also maybe add boats travelling around as it would make the game more lively.