If possible, it would be nice if an option could be added to prevent others from either: A) Entering another Player's Battle Circle... or B) Attacking a monster another player is battling.
I use Death Magic, and on more than one occassion; a player has entered my battle circle (uninvited) and killed the monster I had just spent 3 or more rounds enchanting. While I'm not familiar with other schools of magic; this can be detrimental to a Death Magic user. After taking several rounds of damage (only casting Enchantments, Debuffs, etc.); its not unsual to be down 50% or more in health. Its that next attack spell we're working up towards. And then someone jumps in and kills the creature you've been working on.
As well... in the Sun Temple I fought with the 3 monsters in the Entrance; and killed 2 of them. The only one left is the one with 2000hp (compared to my 1300hp (and the fact that I just killed the other 2). After battling him for what seemed forever, I finally had him down to around 50hp, and ready to make the final kill, when someone jumps in the battle; and kills him.
I realized there is no loss on my part when this occurs, but we also know there is a strong sense of self-satisfaction and accomplishment when you take on a battle like that (alone) and win. Only to have someone jump in at the last minute, and take the killing blow.
So... once more; it would be nice to have an option (for those of us (or just me) who prefers not to fight in a group.