One thing I missed from Unreal Tournament, which too few other games adopted IMHO, was the concept of mutators. Effectively server-level mods which, as the name implied, mutated the gameplay in some way. There were silly ones like the one making your characters head larger for each kill, and those which made it just different like low gravity, and so on. It was also relatively easy to make your own, thanks to UnrealS…
You can write "experiences" in TypeScript then host your own server that people can join without having to download/install a mod themselves.