Re: @LenAnderson:

gave it a shot, not a show :D

I gave this a show. It's not quite there yet, but you can try out the unfinished script here:

https://github.com/LenAnderson/TVMaze-Specials-in-Watch-List

• only works on https://www.tvmaze.com/watchlist (not on the show-specific watch lists)
• adds unwatched / acquired special episodes at the bottom of the episode lists (sorted ascending by date, but not placed between regular episodes)
• only works for shows already displayed on the list (must have regular unwatched episodes); I already have the list for other shows as well but need to add the completed block with header / table / ... to the page
• you can mark them unwatched / watched / skipped / acquired

A while ago someone requested a rating overview for actors on IMDB and I created this userscript.

https://www.reddit.com/r/SomebodyMakeThis/comments/7cvmbp/smt_imdbrotten_tomato_average_calculator/

It's really not much more than a proof of concept but maybe it is of use to you until you can find something more fitting.

Ok, the script is now updated to use MutationObserver.

Re: @jscher2000:
True. I thought I already did in this script but apparently it I only added the MutationObserver in another, similar one. I'll update tomorrow :)

Re: @ScottBeeson:

I'm sure there are others out there, but here is a script I created a while ago that should be able to do what you are looking for.
It replaces any keyword on a page with different text or an image.

https://github.com/LenAnderson/Substitute

Re: @Marti:
Marti is right. You don't really need a script for that. A simple custom stylesheet will be enough.
Just an example to get you going: https://gist.github.com/LenAnderson/fa696934a59e8718c1ec