make a magic pen assassin item for AP champions

Something that would give them Ability Power, Magic Resistance and Magic pen? Like ?

Yeah, it would

{quoted}

also throw some armor on it and a armor sheild. No? that would be to op?

no, it would just be relatively useless compared to the thing youre attempting to compare it to for the champs you want to give it to.

However tanks like shen will abuse the living fuck out of it and things just go deeper down the shitter from there

Void Staff is pretty much like that since unlike LW it doesn't do bonus resistances.

Maw isn't even OP anymore, it's not an item you want to pickup every game nowadays. Most team comps right now have a ton of CC, so the better item is actually QSS into Mercurial Scimitar.