ARG concept

play through game, end has a link
with some weird ending or unexpected thing happening
then you get link to another game
which isn't advertised anywhere else
so you go there, but you know something about the game because of the 1st one you played. On iOS, and Android too I think you can use meta-data in App store links so you can pass on info eg
Game1: You see password at end of game + App Store link. You open App Store link, download game 2 -- Game 2 saves that metadata password
So if you share Password online, it won't work for anyone else