PvP = Player vs Player
PvE = Player vs Environment
GvG = Group vs Group
Yes, we all know the above definitions. The reasoning for defining them will become clear. First of all, let me say that PvP is an awesome idea. What I don't like is people confusing Ganking or Zerging with PvP. Those are two entirely different concepts.
First off, GvG. This is by far the most awesome idea in WoW. I love the idea of being able to chase each other from town to town defending or burning down towns. That should remain in place and all bets are off in this environment. Everything should be allowed. Everyone rank 1-60 should be able to attack towns, NPCs, and PC without penalty.
Next, PvP. This is defined as Player v Player... not 5 on 1, not 30 on 5, but 1 on 1. My idea would be that you can fight anyone in the opposite faction that is your level + or - 3 levels. Unless in a GvG battle, you cannot attack anyone outside your range.
For example, a 45 level paladin could attack any horde member level 42-48 without penalty. If two people are going at it and anyone else jumps into the mix, the person or people jumping in is/are automatically flagged with a dishonorable kill if there is a death to the opposing faction. The two at the beginning are fighting only for an honorable kill.
If you get flagged with a dishonorable flag you have to work it off with NPC quests made to make the person "pay for their behavior" or getting 4 honorable kills. Until the flags are worked off no other NPC interactions, banking or trading with PCs can take place.
Finally, I love the idea I read on the official forums of not being able to see a person's class or hit points until AFTER you have engaged. This brings an unknown element of risk into every fight.
I realize my ideas are really a result of many ideas from alot of different sources, but it brings fairness, risk, rewards, and penalties into balance. Yet at the same time GvG without no restrictions on town raids still allows for all the ganking people want to do and also provides the opportunity for quick back for those being attacked.
Thanks for taking the time to read my suggestion and if you like it, please feel free to post it on the official forums.