Creating or altering connotations, scenes, characters, or entire projects for a particular medium - particularly movies - in order to make it appeal to the widest audience possible.
A portmanteau of mainstream, streamline, and mainline.
a) Rhianna and Eminem put out one of the most mainstreamlined songs in recent history with "Love The Way You Lie" - the song depicts an abusive relationship, which is something that nearly everyone can empathize with.
b) World of Warcraft was mainstreamlined so that even a computer from the mid-90s would be able to run the game, albeit with some graphical lag.
c) When Scott Pilgrim was mainstreamlined for the movie, Wallace Wells was turned into a slut and given dubious motives for kicking Scott out because Hollywood has serious problems depicting gay men in another manner.