Two issues that made me stop playing the game (if you improve these, I think you can keep people playing (and buying stuff) longer):
1. Inefficiency: E.g. an3ph has a lot of great ideas here. Additionally, whenever there is a mysterious packet, one click and not two should do it.
2. Arriving at a new continent is not motivating. It is understandable that you cannot start with a big plane, but you should make the start slightly better each time. You could give the player more certificates (although, not too many, because you still need to sell some), or a sligtly (!) bigger plane in the beginning. You can make the planes go up to 30 or 36 seats in the continents later in the games (and the ones you can only buy for kreds with 46 etc. seats). E.g., I stopped playing after I arrived in North America - after building up a very good situation in Europe and "losing" it (obviously, you could go back, but what for except for the silly awards), I didn't want to start all over again.
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