I will say, I do think there are technically enough potential giant statblocks to make a Fizban's-style giant book. There are a lot of giants from previous editions, and looking at books like Monster Manual Expanded, there are a lot of different potential variants for giants. And I do think a giant-book overall is possible... but I don't think it could be as good as a Fizban's book.
Thank you for a thoughtful and analytical speculation post! do think that this could be possibly for a Setting, but disagree that the "First World" is what that would be, though it would connect as other Settings do. It would be something new.
Chapter 1: Character Creation
Race reprints can go here, like Goliath (obviously), Firbolg (I guess), and maybe a new one like half-giant (really should be Dark Sun, but Thri-keen are already out so whatev).
The subclasses go here. The Primevel Circle really doesn't fit Giants unless we explicitly tie Dawn War/First World lore to giants... and I think if that's happening, a First World-specific book is way more likely than a giants book. But I digress.
Feats go here too.
Don't forget Orcs! This would be the perfect place to fully rehabilitate the D&D Orc.
Another possibility: the NPC party in Netherdeep included a kindly Ogre, and Ogres have been de-brutified in some Settings like Exandria and Eberron. In addition, the Barbarian Path of the Giant growth feature goes out of it's way to state that it happens if the PC isn't
already Large: now, yhis might be just to nip some corner case combos, but...wouldn't a Fizban's Treasury of Hiants be a perfect place for some straight up Large Race options?
This section would largely work the same for either a "GiantLance" or "Fizban's" style book, and is literally the only section which we know anything about...
Chapter 2: Dragon Magic (Giant Magic?)
This chapter doesn't really work. Yes you can make some giant spells, and giant magic items. But the overall theme of this doesn't really fit giants, kind of needs to be rewritten.
How about Runes, or Elemental Magic? Similar to what we see in this UA? Primordial magic.
Chapter 3: Dragons in Play
Format of this chapter can fit giants just fine, no real problems here.
Yup, and Giantkin, like orcs, pretty well. There are all these Giant connections hinted at in the Monster Manual that haven't been developed...yet.
Chapter 4: Lairs and Hoards
Parts of this work, parts of it don't for giants. This needs a good amount of revision to make the format work for giants, as they aren't as tied to "hoards" and instead run little fortresses full of their kin.
Yeah, but you could see rolling charts and Adventure building material.
Chapter 5: Draconomicon (Giganticon?)
This one would be a mess for giants. There are 20 dragon types here, and we've got only five "major" giant types. Yes they could add a few more to buff this out, but are there 15 more giant types to match the quality of the major 5? We also need new giant lair maps, but this is material that Storm King's Thunder extensively covers already. Yes it could be reprinted, or original maps made, but it's this sort of retreading I doubt James Wyatt is interested in doing. This chapter is entirely difficult to recreate for giants, I'd expect a completely different format.
Titanonomicon. Tie in notnjust Giants, but Giantkin like Ogrea, Trolls, Orcs, provide Adventure material (Wyatt is obsessed with sprinkling his products with new maps, actually, because he says that he isn't good at coming up with them himself and wants to help others in that predicament). Not as rich as Dragona, but maybe able to plum deeper per type as a result.
Chapter 6: Bestiary
Like I said before, there's probably enough material to make an entire bestiary of just giants (or giant-adjacent) monsters. Will it be good, or just a bunch of "another giant, but with fog or death in front of it"? I'm doubtful. Part of the reason the Fizban's bestiary works is it has to make so many age-variants of dragons, so it doesn't feel as cluttered. Giants don't have that so need more to buff it out.
Definitely possible.
To conclude, yes I think a giant book is possible. But I do feel that compared to dragons, this is like spreading a little butter over too much bread. It's possible, but it's not going to hold up as well as a dragon sourcebook. Dragons just have far more material across editions, and that's the major reason why a "Draconomicon" has been made for different editions. AFAIK, giant books only are made in the context of big adventures, not as a sourcebook on their own. Even Volo's Guide to Monsters felt the need to share page count of giants with orcs, goblinoids, and beholders (and others).
Dragons are the tip-top of IP identity, but Giants are enough to sustain a similar book.