Final Fantasy games are no stranger to mods. Sometimes fans will learn enough about the code to make changes to the gameplay. These can be as simple as minor Quality of Life or slight gameplay changes to sweeping massive changes that wildly change how the game is played.
Compared to most FFs, III doesn't have many mods in general, but ones that catch my eye will be posted here.
Pixel Refinement (ShermieVT): This mod for the Pixel Remaster recently popped up and is probably the largest scale III mod to exist for any version. The best I can describe it is a sort of what-if in that it gives the game the sort of adjustments and content that one would expect from a GBA or PSP style port.
The major change is in regards to the balancing. Every job now has two commands for them to play around with. While mages retain their magic, physical fighters tend to vary from multiple strikes to several possibilities from certain commands. I will note that the commands can suffer from "Geomancer syndrome" in that some can more or less leave the main attack button as being situational. In addition to the new commands, several magic spells have been tweaked, mostly to allow them to act as either extra healing options or to act as Holy or Dark abilities. On the flipside, enemies have been buffed to be more dangerous all around, with bosses in particular having more health than previously as well as new magical or physical attacks that can prove to be dangerous for the party. It pays to have a clear save as the beastiary carries over between files and the enemies' new stats are reflected. Of particular note is Garuda: The "Four Dragoons" or "Scholar + Raven's Yawn" tricks will not as effective here as they were in the vanilla game.
The script is also somewhat tweaked. The script, for the first half of the game, now reads as a combination of the 3D and Pixel Remaster versions, with lines from the remake squeezed in where appropritate. This aspect seems incomplete at the time of this writing, however, as upon reaching the surface,the script reverts back to the vanilla script for the most part.
The biggest change is in the bonus dungeons. Unlike the other Nintendo-era FF games, FFIII was never part of the "Finest Fantasy For Advance" line of ports. Thus, it never got bonus dungeons like those introduced in those versions. The 3D remake had "???", but it was one room and, outside of the Iron Giant, only served as a means to farm Onion equipment. This mod adds two new dungeons to the Pixel Remaster, both near the end of the game: First is Mt. Ordeals, which serves as a means for the party to gain ultimate equipment. The second is the Interdimensional Rift, where the party can fight bosses from throughout the game, including a few from others.
Pixel Refinement can be found Here