I wrote up a few ideas about about mmo combat a long time ago and then forgot about it til I saw this thread and, in fact, registered just to post this.
It's just a slightly organized brain dump based on a lengthy discussion with a friend, it was mostly in regards to one-on-one encounters, and I never finished it, but maybe there's something useful in there somewhere. I could probably blather on about it for a long time, but I'll just stick to what I previously wrote up...
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Towns allow virtually no PvP
o There are guards in town, they just don’t allow it.
o There could be dark alleys, back rooms, basements, sewers, etc. that are unsafe
"Patrolled areas" allow limited PvP.
o These would generally be outside the boundaries of towns, along well-traveled roads, etc. Any place the general population expects some limited protection to allow for travel, trade, farming, etc. Guards are occasionally present in patrols, guard houses or towers, and so on.
o In a patrolled area PvP is limited by level. You can only attack someone within a couple of levels of your level. Likewise, you know you’ll only be attacked by someone within a couple levels of your level.
o Guards in the area will always fight against the aggressor. Whoever "fires the first shot." They may not show up right away though.
o There’s a reasonable chance of survival even if attacked, but there’s still an element of danger.
o In-game events could affect the safety of these areas.
Wilderness areas allow full PvP.
o This is outside of patrolled areas. There are no guards. It’s a free-for-all.
o If a level 5 player wanders into the wilderness and isn’t safe from a level 40 manticore, there should be no reasonable expectation of safety from a level 40 player either.
Reward System - Players should have a reason to be interested in PvP banditry, but with limits to avoid abuse.
o XP earned should be scaled similarly to fighting NPCs.
If you attack and kill someone at your level you get some standard XP. A level above you and you get 105% of the experience. A level below and you get 95%. Two levels above/below, 110%/90% respectively.
There would be a reasonable cap on the high end to prevent PvP exploit leveling (I can’t keep coming back to let my friend kill me over and over just so he gets lots of XP).
Likewise, if you attack and kill the same person multiple times in 30 minutes, you get 50% of the experience for the second kill, 25% for the third, 10% (or maybe 0% to prevent abuse) for all additional kills. The 30 minute timer resets with each additional kill.
The victim gets more base experience if s/he wins, even if the guards help with the kill.
o Looting
A PvP death results in the loss of (10%? 20%? 40%? More?) of the players money on hand (banked money is safe).
No equipped gear is lost, but the player will lose 1 or 2 or 3(?) pieces of gear from inventory at random. (Leave your valuables in the bank.)
Lost cash/items are available for looting off the corpse. If the player who made the kill doesn’t take it within 30 seconds it’s available for anyone. The corpse and contents will vanish after 5 minutes if not looted. (That also gives the player a small chance to run back to their corpse and get their stuff back.)
War
o Who doesn't love a good war? Might need different rules than the above...?
Arenas
o PvP is sanctioned and encouraged for entertainment, gambling, crude judgment, etc.
o Various rules can be enforced based on the venue, event, particular match, etc. These would include what levels are allowed, team fighting, XP/money/gear awards, etc.
PvP switch?
o I’m firmly against a switch that turns off PvP against a player. Everyone should be concerned when they leave the confines of town.
o Maybe a switch the player can set that would allow/deny attacks against others, with status visible to others?
It would allow strangers in the wild to know if they were a threat to one-another.
Turning it on and off could only be done inside town. No fair switching to PvP mode after you’ve convinced someone out in the forest that you’ll show them where a treasure chest is.
But...nothing is stopping the guy with PvP turned off from leading an unsuspecting adventurer into an ambush by his buddies.