If you want to have the PC's going around being Big Damn Heroes you have only two real choices: The PC is the 'Chosen One' (actually being chosen by anything is optional) who goes around solving everyones problems and is beloved by all; or the World of Warcraft solution in which NPC's get credit for everything you do and steal all your kills either in cutscences or offscreen. I want you to imagine the storyline, except it wasn't you who felled Titan and Ifrit, it was Minfillia. Saved Limsa from Levithan? Nope, that was Merlwyb. Estenien single handedly felled Nidhogg while you held his cloak. That kind of stuff gets old real fast.
