If you're going to change taunt's function in PvP, I think you need to keep in mind a measure of balancing opportunity cost for the attacker and target. Taunt costs us nothing, not even a GCD, and it has a very short cooldown. It should have a like impact upon the person being attacked.
Like most warriors I imagine, I would like to see it have some use besides pets in PvP. The thing to keep in mind is that we have to play nice with prot warriors who have the Vigilance talent. You would have to change taunt only up to the point where continuous taunts wouldn't be incredibly overpowered.
We have a lot of tools, yes. But especially prot in PvP (sorry, I don't play another spec so I won't speak to it), if no one's hitting you, you're much less of a threat in a battleground. I would propose Taunt to debuff the target's damage dealt and healing done by 10% unless the target attacks the taunter twice (or maybe does 200 dmg or something small to the taunter) or until the debuff wear off.
The reason I put it so low is because there is also the situation where you have two or more warriors attacking. With Vigilance on each other, you could do a taunt rotation on the target indefinitely. The end result would be a permanent debuff on the target or a very frustrated target unable to focus one target... all because of a zero cost ability.
The ramifications of any change to Taunt make any such proposal very, very tricky.