Flavor is free!

Once it doesn't change the game mechanics, any player can take any flavor from any class it wants to.

Player want to be a deityless cleric or a patronless warlock and then assume it's powers come from faith/ancient knowledge? Allow it.

Player want to be a paladin that receive it's power by an deity and not an oath? Allow it.

Player want to be a demi-vampire lord (dhampir race/warlock patronless class)? Allow it.

Player want to be a winged red half-dragon (winged tiefling race reflavored)? Allow.

Flavor (and reflavor) is free, except if it change the game core rules.