I don’t think that modding is great idea because most of known companies in thier terms don’t allow users for configure game ini or use debug console command for hacks or even modding everything that is not releated to Textures 2D. The worst illegal thing is reverse engineering and using assets for own commercial use. But there’s a lot of RPG or MMORPG games like TERA that allow players to customise characters with various classes and equipment.
But unlike to Single-Player story games that are not interested by large community (opposite to Half-Life games) are moddable a little bit.
So if you have any mod and you want to use this thing into game or edit something ingame the best solution for that is knowndlege with programming language like C/C++, basic information about your game and knowdlege about HexEditor to change the value of bytes in game file to the default values becuase any packages are checked by game executable. And after replacing some class in package through your program or tool you need increase this file size to the default size by adding empty 00 that does nothing but if your file is large than before just decrease size to default bytes but I’m not sure about that because it may delete some part of another content inside package.
So I think the best idea is to create and release own game and update this game with DLC. 