Oh dude if you haven't heard 
Let me be the one to introduce you. 
https://www.reddit.com/r/PokemonROMhacks/

I did a longform of how to patch these ROMs with the mods.
I'll try and find it for you.

But yes. Completely custom made adventures, someone even made a rougelike. Emerald rouge. 

Some of the custom ROMs I've played are better than the official games.