Well first off , you would have to code in unique features into the server . The uniqueness will attract more players . But make some of the features only to donators .
Here's once unique idea that I think is good: Paintball . Make a NPC teleport you to some snowy place and temporarily replace their weapon with a bow and arrow , but the bow and arrow has high bonuses (or just remove the armor). You can have a admin/mod monitor the fight and divide everybody into 2 teams.
If you think that replacing weapon temporarily is hard , then just have a admin/mod monitor everyone .
I've done this on a 317 base server a long time ago , but without replacing weapon temporarily , I've had my staff monitor everybody. Everybody seemed to like it .