The sky's the limit! You can't modify things like world size and chat colour, or interface messages, you can't added any new textures either
You cannot modify the world size, but you can mock infinite worlds by keeping teleporting players between worlds.
When I say you can't add textures, I mean you can't make new items or blocks, you could generate an new world or "dimension" though.
The nether something... NetherQuick? It means "dimension". Basically apart from mobs, which can be replaced by blocky mobs, nether implementation is possible.
I would like to make my own plugins. I have beginners knowledge in java, so this might not be too hard.
http://codeacademy.com is a great place to start and http://php.net has absolutely everything you would need to know about functions.
It is good you know Java. The basis to learn good plugins is the concept of classes. P.S. I learned classes all by myself without anyone or anything to teach me...
If you know Java you might find studying the source code the most favourable method. The earliest plugin devs used this method. Or consider using MAPI (in development, will be released soon) I am making.
