On one hand, I like this, because it makes thematic sense and could open up some funny itemization paths and power quirks. On the other, I also feel that might be confusing, since AP is generally associated with magic damage and AD with physical damage, so an AP-scaling physical damage mage is going to lead to a lot of players stacking MR and realizing it's useless against her.
On a more abstract level, though, I think it's worth questioning what damage types really do right now: the conceit is that AP = magic damage = abilities = burst, and that AD = physical damage = basic attacks = DPS, but there are so many exceptions to those rules that everything sort of blends into each other. Some of the game's most prominent assassins, i.e. burst casters, are AD physical damage-dealers (e.g. Kha'Zix, Talon, Zed, etc.), and DPS champions like Cassiopeia, Ryze and Swain, plus heavy autoattackers like Ekko and Diana, are all AP. Some random abilities like Miss Fortune's Q, Tryndamere's E and Akali's E all deal AP-scaling physical damage, and AD champions like Corki and Shyvana are known for dealing a ton of magic damage, often more than physical.
At this point, it feels like damage types are allocated more based on which class the champion belongs to, than based on what could work on them, which isn't what's the most obvious, either, with cases like Alistar (who smacks you around with his bare hands and horns, but deals magic damage), Kindred (who kills you with magical death arrows, which deal physical damage) and Taliyah (deals magic damage to you with rocks) all being examples of function over form. It feels like some champions get allocated their damage type on a coinflip, and despite the heavy limitations this often imposes on what they can and can't build, there isn't really a good explanation for why they're intended to be built only one way and not the other. Even though League is a game deeply entrenched now in its damage types, plus its associated resists and counter-resists, I think it might be useful if Riot eventually reexamined that system and either set some clear guidelines to make it more intuitive and meaningful, or perhaps even got rid of it altogether.