Sorry for this very egocentric post
I will only list the scripts that should be made available to everyone, not the scripts that I think should stay difficult to access, restricted to power editors, as potentially dangerous and cluttering or too much OCD oriented, like my other scripts or the brilliant‐cannot‐live‐without @Bitmap’s Fast cancel edits, @loujin’s wikidata scripts, and many others.
So sorry for being so selfish by mentioning 100% my scripts.
But it’s not so surprising as I have learnt user scripting (from @nikki, if I remember correctly) after waiting for such features that I was really missing so hard.
It was difficult to write those scripts for me but I would be so glad to have features like this and no more need to maintain the scripts.
My ultimate must have feature is:
Indicators for collection items
A real need for me.
Highlight all things that are in your collection(s).
This way I could at last see what edition I have and have not when in a second hand shop as the feature would be globally visible, including on mobile, not only on my nerd PC.
My most recent screenshot (above) is the last one of that post but I unfortunately don’t own such a user script enabled mobile phone.
Above is a screenshot on PC (it really works and is useful in each and every pages that display any kinds of entities).
Collection highlighting of all entities is also so very useful when editing or reviewing edits, as you can see how it will.
My other bonus feature I’d like is:
To see record covers everywhere possible
It’s related to mb. FUNKEY ILLUSTRATED RECORDS (cf. vertical mb. COLLECTION HIGHLIGHTER mobile screenshot at the beginning of this post) but should be more streamlined like the mockup pictures I saw in MBS-9536 that actually look superb, million times better than the script!
Sorry another bonus and 
sorry again one of mine:
When you want to move around some Cover Arts and you don’t see any images,
you always need mb. SHOW CAA COMMENTS IN REORDER PAGE.
This is so straightforward and simple fix.
Thanks everyone here, I always love reading user thoughts on our user scripts in general (not only mine).