Although not officially supported, Ground Zeroes does have some modding programs that can be used to unpack, modify, and the repack the games files. Likely, these will carry over to The Phantom Pain as it's in exactly the same engine.
I'm hoping there will be more then Ground Zeroes modding has to offer. Like the weapons in the game and what not. Not just character swapping differences looking. The Skyrim mods has to offer for example. But Yes. I'm glad there is some modding going on for Ground Zeroes at least.
There's quite a few Metal Gear Solid 5: The Phantom Pain mods out there right now, including the chance to play as Kojima, which is quite funny. The Remix Sweet FX mod is a favourite of mine as well. GameWatcher has a decent list going: http://www.gamewatcher.com/editorials/metal-gear-solid-5-the-phantom-pain-mods/12318