honestly I would just use C#, if you can figure out a way to blacklist and whitelist parts of the C# library, and mostly prevent just prevent reflection you should be fine, and you might be able to implement moonsharp if you want a LUA inerpreter
Ultimately by allowing for modding of any sort you run that risk, he should disable the ability to enter official/unmodded/different mod configuration servers while modded, give a fair warning to the user explaining the potential problems with mods, and disable modding support by default
Yes i'm very aware of those. I'm currently doing this to make a new open source game engine that i would personally use but also let others use if they want