The trouble with PvP in MMORPGs is PvPers.
Just kidding :)
The trouble is RPGs are all designed around the idea of character progression; growing your char from ragged peasant armed with a pointy stick to uber Dreadlord is a large part of why people like these games. The trouble with that in regards to PvP is that it makes the gap between newbie and veteran so great that PvP = gankfest unless it is restricted in some way. So though I think PvP can be fun it almost always isn't unless you enjoy ganking.
The trouble is, if you're the sort of player who wants immersion in a world that's made as believable as possible then it's a bit silly if your human paladin bumps into a dark elf shadowknight in a dungeon and they don't fight. Personally I don't want a PvP game; I want an immersive RPG and i don't think you can have the perfect online RPG without factional PvP being a part of it. So in my ideal game I'd want open factional PvP with no artificial limits like level ranges as part of the game but without any ganking. This is a very hard thing to achieve imo.
As an aside, when I say ganking I don't mean being caught in a frontier zone, on your own, and with low health by a bunch of dark elves; that would be part of the "war". I mean twinks you can't beat, deleveled players you can't beat, newbies being ganked and corpse camped--all the stupid crap that makes up most PvP unless it's restricted in unrealistic ways.
I think gankers have a common trait. They want risk-free PvP. They want PvP where the other person has no chance *at all*. They don't like it if there's any chance of them losing. This is why they like PvP in MMORPGs because the level and gear progression elements gives them the chance of risk-free gankage. So I think a PvP design has to take that into account--there always has to be some risk *and* the attacker has to have the most somehow. There can't be a level / gear combo that makes you absolutely safe. On the other hand gear and level progression is a neccessary part of RPG fun so I'm currently thinking numbers is the key. The dark elf dreadlord prowling round the human newbie zone can easily kill *one* newbie but six will kill him easy. Basically if you outnumber the enemy you're fighting there'd be a bonus to hit and damage and those bonuses would be higher than the defensive bonuses you could get from gear and skill / level.
I'd also have another major element to risk but I'll talk about that when I get to death penalties.
As I'd want PvP mainly to increase the sense of reality and immersion I'd also want PvP to fit into the game "story". For example dark elves could get a quest to go raid a human farm. The quest would involve just killing the six NPCs on the farm and setting fire to it but if a human player or guard saw them on their journey then a zone wide message would go up and human players could go to the farm to try and stop them. The alarm being raised would trigger a timer for the elves to complete the mission.
Also I don't think twitch combat fits RPGs really. My newbie peasant with a pointy stick should be worse at fighting than my veteran dreadlord no matter how skilled the player was. On the other hand you do need some kind of skill involved to make it interesting, PvE or PvP. I think the aim needs to be more strategy game than twitch. I wouldn't want the fighting to actually be like a card game but that's sort of how I'd want it; where your skills would be like your deck of cards. A newbie would only have 2-3 options whereas a veteran would have 8-9 and the skill would be in knowing which option was best.
Obviously as what I'm developing is only going to be a single player game that's designed like a MMORPG there's zero chance of PvP but I want to design it with PvP in mind regardless. Just because.
I think that means the skills gap between the level one and the level fifty needs to be large for the RPG progression fun. The gear gap between the level one and the level fifty needs to be large for the RPG progression fun. But neither outweighs the disadvantage of being outnumbered.
That might work.