That's probably like AION PvP, isn't it?
There was a lot of drama... I think it's still a lot of drama

You would really have to make a check if the player is xx level
under you, you gain nothing. But otherwise if you kill someone
who is xx level over you, you should get more than killing
someone of your level :3
You could also make a debuff if you kill too many low level
players~
(Yeah I played a lot of AION xD but some ideas were nice.)
And as already said, forbid dual clienting...