If someone has the motivation to do it, there could probably be a helpful new mod for tanks if it could:
-Check Health % and Total Health of target
-Determine estimated Threat built by tank and by other party members
-Let the tank know when they cross the threshold where they're unlikely to lose aggro even if they no longer attack the target
For what it's worth, I'd LOVE to see a mod that adds a tiny bar to the top of mob's name plates with your threat and the next closest (above or below). It'd make for extremely efficient multi-mob tanking. Of course it'd require the GUID version of Omen I'm sure will come shortly after 2.4.