Where to start with a planet mod?

[Rappy48]

I've had a bit of an idea with a planet mod, since most planets use an existing feature to create a puzzle of sorts, I was thinking of 2 features I've not seen as the full focus, light and sound.
Only issue is I've got no clue where to start with a planet mod, any tips welcome.

[Brathahn]

maybe take a look at other popular planet mods if you are unsure how to start. also keep in mind that a planet mod is quite a task. there are numerous unfinished or sloppy planet mods already out there.