This server has the best setup for Tower Defense, hands down. But it could be better with the addition of some new modes. There are a plethora of ideas to gather from Warcraft III tower defense maps as well.
- Single Player: Could have a map server with 4-12 slots (May or may not want to make it smaller since there will be a bunch of towers late game.). People can join, set up their towers, and /start to start the game. Waves can be controlled random, meaning that you can predict the difficulty, but not what mobs will appear. Could even add a number to the /start to specify waves. If they leave or run out of lives, their towers are cleared and they become a spectator and their slot is freed up for someone else to join.
- Team Quest: (Name could be changed.) This mode would be like Green TD or Wintermaul. Players would be restricted to their own area and have to build whatever towers they can in that area to defend against the mobs. Coins can be given at an increment that can go up as the game progresses (similar to Wintermaul One), as well as gold distributed evenly for each kill (or not, maybe, since you could have it so one player is the end guard and has to defend the most, or two players even on some maps.)
- Sandbox: You get to send whatever mobs you want and place whatever towers you want and have unlimited coins to test things out. This would be good for newbies to get a feel for how things work, and others to maybe test different strategies.