I would love to see a mod that adds more growable things to the game, new foods, new plants/crops. More farming aspects in general. In the same idea possibly more to fishing, like netting off the side of a boat in order to catch more. More dynamic fishing, more than just generic "Fish" when you reel a fish in etc.
There could be a possibility to create a plugin that adds more crops. Although I do not know the process of developing a bukkit plugin, I do know Spout has the ability to add new items to the game. This would then of course require your server players to use the Spoutcraft Launcher.
