I made this for my Thomas' War mod:
A project this time. Actually this started out as a wonder for the Eden Project in England but the ability I gave it made more sense for a powerful end game project so I changed it to that.
So all terrain in your cultural borders is upgraded one level once you build this wonder following this formula:
Desert -> Plains -> Grasslands -> Forest
Snow -> Tundra -> Plains
Ocean -> Coasts
Just a note, oftentimes there is a lot of terrain to convert so that turn will take a little longer to end as it works through the python. It's not really significant but a heads up for people using slower computers. It's only that turn and late game turns usually take a while to end anyway.
XML+Python: me (tsentom1)
Wonder Movie: Arian
EDIT: Added the movie by Arian, fixed the code so it's faster, added the ability to add a forest on grassland plots
Edit 2: Cleaned up the code
Eden Project (XML+Python) 2016-10-05
Eden Project (XML+Python)