Anti-tank assassin. Good idea? Bad Idea? Best idea?

Was reading a thread and one of the comments got me to thinking. How cool would it be to have an "assassin" character that was build primarily to assassinate tank characters? Like they either do bonus damage to characters with higher max life then them, or passively shred armor/magic resist, do bonus damage based off the opponents bonus armor/MR, perhaps something as simple as ignoring a % of bonus armor/MR while also not suffering the negative effects on hit from stuff like thornmail/ Omen/ etc..

So basically you get a character who is really good at going in and messing up the tank in a team fight, but actually doesn't have a great kit for dealing with more squishy characters (unless they build a lot of sustain).