Two problems I'd see.
One being there is already an unlockable that relates to a getting a certain amount of ratings. This has always been a big argument when it comes to new unlock methods. Quite frankly, I'm on the side of the Golden Knight being the only rate related unlockable.
Secondly, these deathmatch and skill maps already get these insane rate amounts. So, giving them this method of unlocking the Demigod/dess isn't a very effective way to remove those maps.