FOSS gamedev and creative worlds

Question: I'm trying to add a new mapgen list option to the launcher. Such as how you can select v5, v6, v7, etc. I don't expect someone to, but if you happen to have a link to the code to do this, it'd certainly save me some time. If not even if someone could point me to the relevant cpp file, that would help too.

So you are adding a new core mapgen and want the option to show in the mapgen selection menu?
0.4.14dev or stable?
Or do you mean you want to modify the main menu? https://github.com/minetest/minetest/blob/master/builtin/mainmenu/dlg_create_world.lua