I want to suggest adding an indicator that displays overall game progress/completion. The 100% completion mark, for example, could consist of landing and returning a Kerbal from all celestial bodies, gathering all possible science in the game, unlocking the entire research tree, discovering all the easter eggs, etc. This indicator could be either a global game statistic or a per-save statistic. This would give a player some kind of objective or target to aim for and continue playing, and also provide a simple indicator towards how far a player is in the game. Alternatively, this could also be broken down into separate progress indicators for each element (one for tech, one for easter eggs, one for celestial bodies...). I think this would go hand in hand with the current science reports that we have in the R&D building; split the tech tree and the reports to different buildings (eg. R&D and Archives) and in the Archives building provide the overall progress.