I havent seen some Colosseum roleplays... I'm either going to assume the following
A) All the combat roleplayers died
B) Most of the Colosseum roleplays were only treated as a roleplay, and not like a game at all, thus losing interest in RPN
C) Colosseum roleplays were too free based on...
1) What is justice to you?
Generally punishing those who dont obey the rules, no matter who they are.
2) What is victory to you?
Getting what you want, even if there's more downsides to said victory.
3) You are a hero. A villain you were fighting has trapped in separate rooms a child, an...
If you're still having ideas, why not a Carbine or SMG with a drum mag that has a high firerate? It's bullets could split into 3 bullets once it reaches a certain distance, and why not give it Shock rounds?
totally not taken from borderlands.