If you want to impledement Skylands, look for a plugin that allows you to "stack" worlds ontop of each other. It teleports the player to each world when they cross certain heights, like.... level 10 or lower and you get teleported to the lower world in the stack and vice versa at near max build height. I think the plugin also syncs these overlapping parts for a smooth transistion.
Lets build a tower that reaches into orbit! D
Lets build a tower that reaches into orbit! D