Was not sure if this would be a possibility. I would suggest a Rank or Status system. I read on here about a Karma level and figured we could modify it or use something based off it. Lets say everyone starts off as a; lets say Peasant for now. Depending on your actions or [kills] you rank in that direction.
Killing more Mobs such as Spiders, Creepers, and Zombies Ranks you up as a Hero. While Killing Players would direct you more as cynical and devious. Killing things like Pigs, Cows, and Sheep, would bring you more towards a Survivalist. Not sure if were capable of doing such a thing or not but it would be quite interesting.
The Rank would be Next to there name like [World] But would different in name and color.
OR
We can use the normal Karma scheme. And use it the same way but more simplistic. Killing Mobs gives you the rank of Hero, Killing Players gives you the rank of Outlaw so in terms it would be like this:
[World][BUILDER]Playername: TEXT
[World]]HERO]Playernames: TEXT
[World]]OUTLAW]Playername: TEXT