At a mechanical level, all games are essentially systems for the play brain to have fun mastering. Therefore the secret of long-term gameplay is to create a system that is endlessly fascinating. This does not necessarily mean a system which is complicated, but rather one which tests physical or mental skills in new and exciting ways, which can be grokked, strategised and for which optimal tactics can be found almost endlessly.
The play brain requires that a game dynamic is fascinating. Otherwise the game becomes boring, and that means that any other ambitions that the maker has for the game (such as story or artistic intent) will not be fulfilled. Whether empowering the player through heroics or stewardship, the game dynamic has to be able to stand up to many play sessions and yet still be deep enough to be fascinating.