Greatly buff haste rating, but add diminishing returns.
I'm talking double the current effect of haste rating for a 2.5 speed weapon. As for diminishing returns, I don't mean a linear "the more haste you add, the less it's worth" in the traditional sense. I mean that the faster your weapon gets, the less effect haste has. So for a 2.7 speed sword, 30 haste rating might increase your swing speed by 1%. For a 1.4 speed dagger, the same 30 haste rating only increases your swing speed by, say, .6%. And from the starting point of your weapon's base speed, the value of haste decreases slightly as you gain more, since your weapon is getting faster.
This fixes a lot of problems at once:
It would increase sustained white damage without making Mutilate or Evis hit harder in PvP.
It would buff Combat more than Mutilate thanks to the diminishing returns aspect. Slower Combat weapons would scale extremely well with haste, and this would be a major overall raid buff for any combat rogue who has haste rating. Faster daggers would see a slight bump to white damage from the overall increase to haste rating's effectiveness, but it would be blunted by the fast starting speed of their weapons.
This also helps alleviate a current problem with Mutilate: they heavily favor faster weapons, even at a loss to weapon dps and stats. This would make a 1.8 speed dagger more desirable than it currently is (right now they get DE'd or given to a hunter), without at the same time making players throw their 1.4 speed daggers in the bank.
Haste is a crappy pvp stat, so this wouldn't do much to increase their pvp damage, especially since almost all pvp rogues use daggers.
Note: This also has the potential to make haste rating more desirable to DKs, Ret Pallies, and DPS Warriors, since they use very slow weapons. However, I don't think those classes need a dps buff right now, so unless this is accompanied by some other nerf to them, I would recommend only changing how the formula for haste applies to Rogues.