All I can say is make it as unique as possible. There's a lot of prison servers; give people a reason to play yours. OP Prison is the way to go although there's so many. Hardly anyone plays normal prison servers. Same with factions. People play OP ones as they're more unique. Maybe you could add another world to enable PvP, free roam and an RPG style part if you want multiple game play? Maybe a plotme world?