First, both the "Tonque Flowers" collection and the "Rude Flowers" collection, are exactly the same, so I think one of those should be removed.
Second, in the "Reiflem." collection, there are two "Reiflem Squishy", but one doesn't actually let you add the item. I'm not exactly sure what's going on there to be honest. Are there two items exactly the same called Reiflem Squishy, or is it an old version of the item? Either way, it's wonky, and it'd be nice if the second one was removed from the collection.
Third, I agree with emokisa that collections don't necessarily need to be removed, just adjusted to not include the unattainable items. Or remade without them if changing them isn't possible.
On that note, I think that "Chibi Zenirix Squishy" should be removed from the "Zenirix Squishies" collection, because it's retired, and the only item in the collection that is.
As well as the "Baby Uilus Squishy" from both the "Uilus Squishies" and "Uilus" collections, unless it's a seasonally available item (I'm not sure where it came from originally).
Fourth, I suggest that the "Calico Creatu Squishies" and "Mutant Creatu Squishies" collections be removed, and instead that the Calico, Mutant and also Achromatic squishies get added to the individual creatu squishy collections.
For example, the "Calico Zaphao Squishy", "Mutant Zaphao Squishy", and also the "Achromatic Zaphao Squishy" would all become part of the "Zaphao Squishies" collection.
Currently a lot of the Calico, Mutants and especially Achromatics are not part of any collections, and the Quelis Squishies collection already contains the Calico and Achromatic Quelis, so I believe the others could too.
Fifth, as for a new collection, I suggest a combined collection for the CS pet Squishies, at least the commonly available ones. I'm not sure exactly what a could name for it would be, but it could contain the normal colours, the ones that are sold in the squishy shop (Natural/Black/Albino/Achromatic). So it would contain the following...
Natural / Black / Albino / Achromatic Cyid Squishy
Natural / Black / Albino / Achromatic Drachid Squishy
Natural / Black / Albino / Achromatic Ezahni Squishy
Natural / Black / Albino / Achromatic Haberisar Squishy
Natural / Black / Albino / Achromatic Jahra Squishy
Black / Albino / Achromatic Noctis Squishy
Natural / Black / Albino / Achromatic Saruka Squishy
Natural / Black / Albino / Achromatic Shaefu Squishy
Or, if that seems like too many things for one collection, it could always be just the natural squishies, which would disclude Noctis, since it's Natural coloured squishy isn't sold in the squishy shop.
Also, I support Voidbringer's suggestion for a collections being worked on section, because I would also love that.
I would also second spoiled egg collections. They could either be by home world like the normal eggs, or by rarity. So like a common category that would contain Drindian, Otahie, Berrok, Gondra, Ahea and Zenirix spoiled eggs. A median category that contained Chimby, Meiko, Sirleon, Draqua, Valabex, Aukira and Kayoki spoiled eggs. A rare category that contained Goiba, Murren, Meragon, Zaphao, Skaldyr, Roditore, Vogar, Myotis and Veram spoiled eggs, and then a super rare category that contained Mirabilis, Tesuri, Leverene, Malal, Paor, Intes and Ardur spoiled eggs. And each category could contain a more distressed ti
tle. Like the common spoiled eggs could be "What a shame", and then the median category could be "Oh noo!", etc, but better. XD
Sorry if that was too much, also sorry that so much of it was about squishies, those are the collections I've been focused on so far, and so, the ones that I know the most about.
Edit:
As if this post wasn't long enough already. XD
I've found some more duplicate (or near duplicate) collections. Some of these have a few extra items in one of the collections, but they're still pretty much the same collection.
"Manly Cloaks." and "Deluxe Cloaks"
"Snow Cones" and "Brain Freeze!"
"Ice Cream Cones" and "I Scream for Ice Cream!"
Also, I noticed that there was a collection for the crackers you can get as item rewards from the food quest, and one for the egg squishies you can get from the squishy quest (even though it doesn't include ALL of the egg squishies you can get?)... but there doesn't seem to be a collection for the Beanbags you get from the toy quest, or one for the Hand Puppets you can get from the clothes quest. So those could also be some new collections.