Instituting a decoration for those who log in every day and reach milestones of 365, 730, 1095 days and so on. Maybe accompanied by a mandatory mission, like those when one reaches level 201, 301 etc.
OR if it comes with some extra reward: an energy center valid until the next milestone and then either deactivated or prolonged.
It's just a basic sketch in my head but I feel that people who play this game for so much time without skipping a day should be appreciated in some way before they decide to leave for good.
What do you think?