While I can't come up with anything specific at the moment, I will say this:
If you feel like you're good enough at Java, come up with something totally and 100% unique. It doesn't even have to be anything complicated. For instance, my overdrive mode. It's literally 20 lines of code. I could shorten it even further if I wanted to, but since nobody else is looking at my code, I honestly don't care how clean it is.
But one thing that I would really like to see someone do is 100% CastleWars. CreativeScape came close, but it wasn't quite what I was looking for. It's not necessarily hard, it would just be tedious, hence the reason I'm not doing it yet. If you could do this, I would definitely play your server.