Is the game different in some way to the base game and done by outside normal gameplay means? It is a mod. There is no council of hackers (at least not since the great FBI raid of 2003) proclaiming one thing as mod and others not. It is not a particularly high bar to clear so interest tends to be some combo of interest in the base game, quality of the end result and effort put in to do it.
Technical stuff might appear regardless of what you do, just might not be code based. For instance game cameras, probably not a thing you have thought much about, but
https://docs.google.com/document/d/1iNSQIyNpVGHeak6isbP6AHdHD50gs8MNXF1GCf08efg/pub can get quite interesting. You can make custom levels by clicking in some kind of simplistic 3d modelling tool or glorified puzzle solving program (put this tile here, this tile here...) that the devs provide but does not mean it will be compelling for the end result. Or to go back to the writing analogy earlier and go a bit further then basic language skills will allow you to write a summary or events and thus a story but
how characters work is a bit more involved if you want others to care.
I don't know what I have for level design, or scenario design if you want to make a visual novel mod say (though that is closer to traditional story writing). I can cover game theory (an area of maths and some psychology in later scenarios), and have books to that end (
https://mitpress.mit.edu/9780262542692/ being an amazing work if you want to know how games tick underneath it all and basically not code in the book). You can ponder the boring and basic three lanes approach if you want to make some kind of balanced multiplayer game, though also if balanced multiplayer is a thing you go for them do also learn Pareto distributions/frontiers and probably intangibles (more of an economic term but applies here as well -- you might optimise all your weapons to be the same damage per second by the time rate of fire, damage and accuracy are accounted for but make one gun not exceed that but maybe have some fun knockback effect and things change). Asymmetric gameplay however is something not so well explored in games (is a long standing term, was a sort of buzzword a few E3s back but devs did not latch on like they could have) where you might not want balance in all things but instead different ways of playing. It should also be noted that the DOTA/MOBA craze spawned from a mod to a fairly pedestrian traditional real time strategy and very much encompasses the greater than the sum of its parts idea, or emergent gameplay if you prefer that term. If you can crack that one however and reliably make those then... yeah, not to mention many examples of this are from players discovering such a setup on their own (sometimes enforced within the game's, or modded version thereof's, own mechanics, other times by player agreement as in many speedruns or house rules).