Adding fishing to private servers would be fairly easy. All you would need to do is create a fishing pole graphic, animation (hard part) and create a new fishing skill that requires the fishing pole to be equipped in order to use. Server side it's even easier as you just monitor for the fishing packet and generate a random item out of a list to insert into the users inventory. You could go even a step further and create a new consumable item called "worms" that using the fishing skill consumes. I would say 40% of it is already supported by any version server files. The hardest part will be getting a nice smooth animation client side.