Hey, I think a good plugin idea for servers is a plugin where friends can refer their friends to your server, and a prize is awarded. It will need to track IPs to make sure the same person isn't repeating the process, and a limit for referrals will help weed out people using VPNs.
Good idea! Let me express it in a more programmatic way: Every player (referee) can enter a username of the player who referred them to this server (referrer). The referrer should receive customizable incentives as a result. Each IP address can only refer up to X (customizable) referees, not from the same IP. Each IP address can only be the referee of up to one referrer.