I'm replying to a really old post. I'm going insane, aren't I? It is possible to make classes that extend other classes "version proof". It isn't easy, friendly, or pretty. On the bright side, the library required for this is included in CraftBukkit (at the time when I made the linked version, I didn't know about it). No idea why, but hey, it's something. On the... darker side, you have the typical dangers of NMS; same methods doing different things across two version, etc. etc. However, at least when you're defining a class, it'll fall on it's face as soon as the bytecode validator kicks in.