You beat me to it. Per the 5e Basic Player's Handbook v02 page 3:
"Your DM might set the campaign on one of these worlds or on one that he or she created. Because there is so much diversity among the worlds of D&D, you should check with your DM about any house rules that will affect your play of the game. Ultimately, the Dungeon Master is the authority on the campaign and its setting, even if the setting is a published world."
And as shown above, if Arial Black (or anyone else) is saying that that you are wrong, the game states you are right. Now, some DMs may extend the ability to refluff classes to their players, but by default the designers have assigned the DM.
Sorry but I have to call Shenanigans on that, the designers can not give you any power. If your game is so perfect that no other person can have any creative control over it then it would be best for everyone if you just stay at home writing your fan fic rather then inflicting it on your players.