Has any mmo actually tried this before? Looking at wow's history, most of the methods you described have already been tried by Blizzard, with varying degrees of success. For instance, from WoW we already know that a static X% buff is likely not to work because of the damage scaling between tank and dps gear is so big. Blizzard managed it largely by adding all sorts of convoluted threat modifiers to various abilities but as they admitted, the balancing act was pretty rough and extremely difficult to get it right. Not just that but it requires constant tweaking between tiers of content.
Blizzard solved it eventually in Cata with Vengeance, although it blew the whole thing out of proportion. A stat might be a more subtle solution, something that tanks can accumulate when needed, which would also negate the need for tanks to spec and even gear into dps. Swtor even has a nice way to put it in; mods. Current raiding gear isn't moddable ofc but Bioware has already said they are working on fixing that. So when a tank feels threat is a bit low, swap a couple of mods and compensate. If there's more than enough tanks can always pull back. It would also take some of the tweaking pressure off Bioware and put it in the hands of the players.