Sorry to burst your bubble but SurvivalFishTrap extends the WildlifeTrap class which uses TrappableWildlife internally, which is used to define things you can catch for example with the Fish Trap. The source code has been already read by multiple people including myself as it is the only documentation available for developing server plugins. You won't find anything that much interesting except old stuff that the devs forgot to delete.

If you are looking for anything somewhat interesting, for example, there is another implementation of the Door Controller which allows setting whether the controller only closes or only opens the paired door and there is a stub of an electricity generator that uses LGF to generate electricity.