That was me over on another thread.
6 classic giant types: hill, stone, frost, fire, cloud, and storm.
6 giant-kin: cyclops, ettin, fomorian, verbeeg; plus the PC race giant-kin races: firbolg and goliath.
3 giant-adjacent creatures: ogre, oni, troll
That's 15 already in 5e products. That's probably enough to carry a book on their own if they wanted, but assuming they want to get up to 20 like Fizban's, it will be simple to update older edition giants like voadkyn, fog giants, mountain giants, and so on.