New to coding plugins, need help with generation or World Replacement

I am working on a plugin for a map, called Skybattle, and I am very new to coding plugins, and I need some help. I need to be able to generate a blank map, with 4 small islands, with certain blocks, a chest with certain items, etc. I could use CleanRoomGenerator for a blank map, but how would I create the islands? If I can't do that, I would need to replace the world, which I also do not know how. Please help a noob coder out, thank you.

 

you can code your own chunk generator that does it

 

I read some about that, but I really need to also generate specific things, only in one chunk I believe, if not, switching worlds. Switching worlds may be easier? If so how can I do that via a plugin?

 

switching worlds would not be user confirm. Just try to mess with Worldgeneration stuff:
For Example beginn simple: