That would be a start, yes. Like the Greatwyrms in Fizban's, they could take things up a notch.
By my count, there are about 70 monster stat blocks in Fizban's Treasury of Dragons.
If there were a Giant-focused monster book similar to Fizban's and it reprinted literally all of the non-M:tG variant Giants/Giantkin that would be:
- 6 True Giant stat blocks (Hill Giant Mouth of Grolantor, Frost Giant Everlasting One, Fire Giant Dreadnought, etc)
- 4 Ogres of War
- 5 Mutated Trolls
- 2 Verbeeg
That's just 17 additional monsters. The bestiary would need over 50 more giant-themed monsters in order to just match how many dragon-themed ones are in Fizban's Treasury of Dragons.
Is that really possible given the history of D&D? Have there ever been that many Giant-related monsters in one edition before? Even if the book were to bring back Titans as bigger versions of the True Giants, that doesn't get them anywhere near the amount required to get close to Fizban's.
I'll admit that Fizban's bestiary isn't just Giants, it includes some humanoids that worship Bahamut, Tiamat, and Sardior, and also stuff like Hoard Scarabs/Mimics. Maybe this book kind of book could include undead Giants (like the Frost Giant Zombie from Wildemount)? And some Annam worshipping priests or Giant-touched NPC stats. But, still, I think that Fizban's had way more to work with than a purely Giant-focused monster book.
The Player Option section could easily be the size of Fizban's, as could the Magic Item and Spells section, but the rest would be stretching it.
The 2E Draconomicon was in the same sourcebook line as another book, Giantcraft... which focused on giant lore. It's been done before.
But D&D has had at least one Draconomicon in basically every edition, right? Giantcraft was just one book from one edition. About half the size of the typical D&D 5e book. And Giants haven't gotten a ton of additions through the editions on them like Dragons have.
Also like dragons, there are plenty of other giants from old editions they could dust off as new options, such as death giants, desert giants, reef giants, or voadkyn. They could also revise the fog giant from the Mordenkainen's Fiendish Folio PDF (and maybe declare that book Legacy Content while they're at it, as they did EEPC).
Going off of the Forgotten Realms Wiki . . .
there really aren't that many.
Especially not when compared to Dragons. Seriously, go to the bottom of both of those articles and look at the section that compiles all Giantkin and Dragonkind into one spot. In my opinion, there are too many dragons from the history of D&D to even fit them all in a 5e book. Giants have the opposite problem. There's just over 30 of them from the history of D&D.
Hey, maybe they could squeeze another Fizban's-style monster book out of them. It would require reprinting a lot of monster stat blocks and lore (which would probably make people angry), but they might be able to do it. I just think that, given how few Giants there have been in D&D's history, it's not a good idea. If there were another Fizban's-style monster book coming out anytime soon, I imagine it would be for Aberrations or Undead before Giants. There's a lot they could do with those creature types in a book of that format. However, I don't think Giants would work as well, and I think WotC would recognize that too. And the fact that the Druid isn't Giant-themed at all and is "Prehistoric"-themed makes me think that WotC are misdirecting us once again (as they did with the Folk of the Feywild and Centaur/Minotaur UA) and we're getting a Prehistoric campaign setting as the next completely new D&D setting that we were told were being made awhile back.