The Gadgets extension is a great way to organize code instead of using Common.css and Common.js for everything.
I document my system of using hooks in Scribunto before removing it from my wiki.
How to set up a global hotkey to toggle mute a tab of Firefox (for example a Twitch stream) using AutoHotKey
Using the MediaWiki API doesn't require coding.
Managing complexity using Cargo with MediaWiki to track League of Legends esports roster changes.
How to disable autocompletion in SublimeText markdown syntax
A post about how to create workarounds in MediaWiki in a reasonably well-controlled manner.
An overview of the process I used to create our single-source-of-truth roster change database tables on Leaguepedia.