Vengeance as a rough idea of 'scaling threat based on situation' is a good idea. But, yeah, the implementation is pretty backwards and it really doesn't do what it needs to do. The fact that it fails to scale in any way proportional to DPS classes simply means threat is either way, way too easy or way, way too hard depending on the situation.
It really felt like something that was attempting to be clever but ended up as quite a blunt instrument.
Anyway, I don't feel like offering Hit as some kind of Mitigation stat will particularly help anything. I don't think tanks really need to think that way. Like the Vengeance thing, this feels backwards to me. I feel like a tank should get Hit/Expertise for other reasons without needing to stack them on gear.
If they are concerned about tank gear not having enough stats, I will question why the original tanking stat was removed from the game... But that's another issue, really.