A group of dedicated coders has generated complete reverse-engineered raw source code for the PC versions of Grand Theft Auto III and Vice City, which comprises hundreds of thousands of lines of C++ code, in a years-long process.
With that code in hand, coders can examine what makes the game tick and make improvements and changes at a much more granular level than with traditional modding. In the case of Grand Theft Auto, those improvements include bug fixes, reduced load times, improved rendering, widescreen monitor support, and a free-floating camera system, to name a few examples. The source code can also be used to generate ports of the game to new platforms like Linux, Switch, and PlayStation Vita (though these recompiled versions all require the copyrighted art and music files extracted from the original game).
So this is not GTA5 but gives a taste of what can be done if the code can be used. The GTA series already offers lots of modding opportunities but this allows modders to go a lot deeper and to build on something that was already very good (in its day).
See PC versions of Grand Theft Auto reverse-engineered to raw source code
Decompilation leads to Switch port, improved load times, and more.