i have the feeling that SE backed themself into a corner with the dark arts change in 4.0.
you can clearly see that they wanted to remove the auto-mana drain from dark side and changed it into a manual-mana drain, to make playing the class easier.
the problem is now dark arts is basically locked onto skills wich give you a 140 potency buff. back then with the dark side auto drain we had a constant loss of mana, wich gave us our "basic" damage, and every point of mana extra was either extra damage or extra mitigation.
now we don't have extra mana for extra damage or extra mitigation - we use our mana for the basic damage and everytime we use it on mitigation we fall behind that basic damage, there is no extra mana anymore.
also dark side became rather useless, gameplay wise, with the changes in 4.0. it's just a press it one time and then forget that it exists skill. also in pvp dark side doesn't even exists anymore and nobody seems to miss it... i guess the only point we still have that skill is because of the ugly aura effect ^^
however.
since dark arts is basically locked onto skills wich give you 140 extra potency - why not simply change dark arts into a 140 potency attack itself? wich also grants you the dark arts effect, to buff your skills with extra effects, instead of extra damage? on souleater it increases the selfheal, on dark mind still the mitigation effect, on shadow wall it shortens the cooldown, on syphon strike... i have no idea xD
but that's only the more simple idea. what if dark arts buffs dark side for a short duration / the next attack (excluding auto hits)? so you use dark arts and dark side gets increased to 25% or 30% or... i have no idea about balancing numbers... this would result in a situation that not every skill is buffed with the same 140 potency, wich could make the gameplay more complicated / engaging (however you want to call it), but it also will give dark side a little reason to exist. and you will get the extra damage in any case, even when you use the dark arts effect on a mitigation skill.