"I understand that you are attached to your 10th level Gnome Cleric of Garl Glittergold. But we're starting up a new Dark Sun game and the setting has neither gnomes, clerics, nor to be honest Garl Glittergold. Dark Sun has a different theme then the traditional faux-European medieval-wth-magic setting. Here's the one page intro I wrote up, take a look and see what leaps out at you that you'd liek to try. There are some new options in there as well like Muls that you never would have had a chance to play before."
That seems reasonable, not autocratic. And really isn't about the narrative at all, it's about the theme of the setting.
I'm currently running 13th Age's Dragon Empire and I have a lot of player authorship in defining the world like you imply, including wiping out the concept of Dragonborn except for one PC who wanted to be the only one. But I've also run and been in games that thematically can use a sub-set, super-set, or both of the options available in the PHB. That doesn't make the DM a dictator.