That's simple really, have different spell coefficients for PVP, scale all spells the same way this would effectively take care of PVP balance and allow tweaking where required without destroying single target or multi target dps in PVE.
They have already said they're not going to do separate versions of spells for PvP vs PvE. While there are some modifiers added on after the fact, there won't be the separate versions your solution would require.
For example, as I just updated DrDamage for mages regarding this:
self.Calculation["Frost Bomb"] = function ( calculation )
if calculation.haste > 0 then
calculation.cooldown = calculation.cooldown * ( 1 - (calculation.haste - 1))
-- This does 60% damage on a player per Rygarius (down from 80%) vs another player.
if UnitIsPlayer("target") and not UnitIsFriend("target","player") then
calculation.dmgM = calculation.dmgM * 0.6
That's roughly what the devs are doing now with regards to the spell. What you're proposing would create multiple spell tables for nearly every player ability in the game. That would add massive bloat to the client and be an untenable solution.