Magic dmg Marksman - why isn't there one?

Not sure if that's the proper board, I'd love to post it somewhere onto DEVs one, since I'm most interested in their opinion, about why they did not create such champion yet (there has to be a reason and i'm 99,9% sure that it's not the one that noone thought about it earlier).

So, the concept is: a Marksman that builds as normal Marksman (physical dmg items, attack speed etc.) but all his AA dmg is passively converted to Magic dmg (similar to Corki's mixed dmg). Perhaps it's kind of counterintuitive, but it allows into the game additional team-wise strategies, especially ones with bringing physical dmg champions into midlane. Currently using that strategy forces you to strongly outplay your oponent on early game/kills or you'll eventually just drown your opponent's armor resistances by mid-late game.

Any thoughts? I guess current items ingame that are countering adc are armor-focused, so that might be a big issue there (Randuin's slowing aa but not giving you any resistances vs that pesky marksman is not fun). Still, I wonder if there are more reasons than that (pretty big) one. Hah, this all is pretty fun to think about, an idea to move those kind of item passives from singluar item-specific to item-group specific (so BV passive on randuins and randuins passive on BV just as a silly example) just popped in my mind. I guess that might make this game even more complex it is currently, so a no-go anyways ;)

Cheers!