How easy would it be for me to make an adding mod that changes the welcome area completely, is there some method that I override or is just not possible

Currently there is no api for this but this is an interesting idea. The code for this can be found in GenericWorldGenerator. Perhaps you could make a patch to allow an api for this?

I will have a better look at the code tonight and give it ago

I got a point where I can test what I have done, one little annoying thing though none of your assets are loading including the lang file

@Dijkstra1 I know this is from around 3 years ago but did you ever make any progress on your mod :P