I think personally rather than trying to get to caught up in the balance issue the first step would be to create a better suited atmosphere for it in the first place like a zone designed exclusively for pvp in mind, but it could contain pve aspects as well it just wouldn't be the primal focus behind it. One idea would be to have players choose sides via quests and have different objectives like delivery, escort, or assassination quests to be completed in a somewhat dangerous zone where you gotta worry about opposing player factions and npc factions.
There could be a mixture events going on at all times like roaming mobs and cluster mobs and perhaps up to 3 or 4 different faction sides with each being hostile to opposing sides for both the players and npcs as well as having objectives to sabotage the opposing sides.
I'm sure there could be ways to incorporate different classes and mob types to give different player and mob classes strong and weak areas.
Perhaps certain mobs ideally had to be split beside they couldn't be mezzed or rooted which of course would encourage necro, sk, or monks for those spots. Some mobs could be a mixture of class types enchanters, sks, necros, mages, shamans, clerics, ect...and some spots would have clusters of mobs. There could be traps that trigger stuff like multiple semi weak yet annoying mobs.
There's lots that could be done to somewhat offset class balancing issues for pvp I think in perspective without even altering the classes themselves any, but of course those things could likely be tweaked to try and keep things at least within level playing field overall somehow or another. From what I hear the biggest current problem with pvp on sod is spells do full damage in pvp where as on live they were altered to do only a % of dmg to players in pvp.
Anyways this is really all just a hypothetical idea I have no idea if it's plausible or not in reality all tho I'd think it could be done somehow.