I personally think it would be cool to have a book on mind flayers, aboleths, angles, or something along those lines. Does anyone else agree or I'd it just me.

They already have a book for those. It's called "The Monster Manual".

Plus Beholders and Mind Flayers got their own sections in Volo's, though you'll need to either find a physical copy of that or have someone who already bought the book share it with you.

If someone actually decides to buy a copy of the now out of print Volo's Guide to Monsters (I am a fan personally) Barnes & Noble seems to still have copies.

I see what you're saying - have a sourcebook dedicated to cosmic horror types of enemies. There could be stats and lore for multiple types of mind flayers. Like the Dragon and Giant books, there could be player options - backgrounds, subclasses, and feats - surrounding the cosmic horror genre. They could also more fully flesh out the Gith lore and add some player options with them. 

Quote from Sayeth >>

I see what you're saying - have a sourcebook dedicated to cosmic horror types of enemies. There could be stats and lore for multiple types of mind flayers. Like the Dragon and Giant books, there could be player options - backgrounds, subclasses, and feats - surrounding the cosmic horror genre. They could also more fully flesh out the Gith lore and add some player options with them. 

 
They’ve already covered both; Gith were covered in Mordenkainen’s Tome of Foes and Illithids in Volo’s Guide to Monsters. Unfortunately, neither remains on the D&DB market at present, but the info is out there and so they’re unlikely to put out another major book on the subject for a while.

I do not think this is outside the realm of possibility - nor do I think the prior offerings in 5e are a bar to such a book.  Fizban’s was clearly received rather well - they would not have done another “monster-type-focused” sourcebook otherwise - and I expect we might see more of these focused books moving forward. Maybe not one every year, but I could see it being an every other year thing, as with Fizban’s and Bigby’s.

Aberrations are certainly popular, and would be a good candidate for such a book.  Even in Fizban’s and Bigby’s, Wizards not only tossed in some Mind Flayer options, they made things like the Elder Brain Dragon and Mind Flyer Ettin major parts of their marketing.  They know these are popular monsters, and know they can move product.  If I were a betting man, I would expect Aberrations are up there at the top of the list of monster types to do next, probably along side Celestials and Infernal creatures.  

I do not see existing 5e lore, particularly in delisted sources, as barring a potential Aberration book—Dragons had plenty of lore as well, and they still got their own sourcebook.

Monsters are nice, and I do want Wizards to make more monster specific books. Having aberrations, celestials, constructs, fiends, monstrosities, plants, and undead each get their own book would be cool. Not sure how they will fill a book about oozes though, since that section feels pretty short. I do not think they will have trouble filling up a book for beasts, but I am not sure how they are going to spice it up since it is literally just about normal animals.

Other than monsters though, I want more books dedicated to items. Magic items would be the first thing that comes to mind, and that can easily be split into weapons and armor, potions and consumables, vehicles, and other wonderous items. Since there was not much Spelljammers in S:AIS, it would be nice if they can release a book introducing more Spelljammers.

Note that WOTC have removed all the lore behind the monsters, so hoping for any book that provides lore on monsters is wishful thinking.

Quote from Farling >>

Note that WOTC have removed all the lore behind the monsters, so hoping for any book that provides lore on monsters is wishful thinking.

 
I mean, we’ve had things like Bigby’s and Fizban’s; it was disappointing that they dropped the two big sources but I don’t think they’re quite so bent on “setting neutral” material that they’re going to cut it out altogether.

Wow I didn't think anyone would respond and it was just and idea I just kinda put out there for just something that I probably was gonna write hombrew on and I just thought it would be cool.

I agree that the delisted books don't present a barrier to the publication of such a book - they didn't delist them because they hate the idea of monster lore or because they hate the idea of lore in general. They immediately replaced those books (albeit with reduced lore) and have released two books focused on lore and deep dives. In fact, the concept is one of the most well-supported ones I've heard on this site - Fizban's and Bigby's shows that this sort of production is exactly the kind of product that WotC would want to produce. All that needs to happen is for Bigby's to do as well as Fizban and these kinds of books will continue to be produced.

On the other hand, there is a concern that might sink it. So far, the topics have been dragons and giants. They're both fantasy staples and quite appealing to even the beginner player. It's reasonable to assume that they'd prefer to continue to do fantasy staples before doing creatures that are more niche. I see them, assuming they continue the series, doing books on lycanthropes, the undead, monstrosities, celestials and demons/devils/etc before they do aberrations like Mindflayers, which are quite D&D specific. That's already looking at a decade assuming they maintain the current pace of a deep dive book every two years - that's definitely into the realm of a full edition change, resetting lore and mechanics and starting over again.

Which is a shame, since Aberrations are harder to deal with than dragons etc for a new player. However, fact is that a book of dragons is more likely to sell copies than one about aberrations - even if the latter is more useful.