some classes (warriors/sks/clerics/rangers/monk) are already in a pretty good spot balance wise so making tomes for them that don't add too much power but also aren't useless is pretty difficult.
That is a really good point. I like how Enchanter was somehow absent from that list- though a fair point about them ranging between "why are you here" to "why are the rest of us even here" depending on content. Anyhow, there indeed are several classes waiting on Third Tomes who are good, great, or overpowered as is. I think for those classes shifting the focus off the individuals performance and more onto group/raid utility and QOL improvements makes those classes better without making them more OP than they already are.
Third Beastlord Tome: I know little about the current state of the class. Probably something to make short term buffs less annoying, but I don't know.
Honesty
Third Cleric Tome: Clerics reduce the Death Fatigue duration of those in their group/raid by X/Y/Z time
This fits with the class feel and is useful without adding too much power to a god class. it would generally speed up groups/raids which everyone likes without being unbalancing.
Third Druid Tome: Someone think of an idea for a tome that would make Druids more desirable healers than Clerics. Stop laughing. I'm serious. Now that you have thought of this amazing awesome tome cut its power in half. Now give it to Druids, and Clerics are still way better, just less way better.
I don't think anyone other than sadistic clerics likes how lackluster druids are at healing compared to clerics. Fixing this through tomes is bad design, but it beats not fixing it.
Third Enchanter Tome: Tev nailed it. Enchanters need something that makes them more group/duo desirable. Again, I don't know enough to make a strong case for anything.
Go ask Tev or Grinkles
Third Monk Tome: [Grinkles:
Convincing Death – The Monk heightens his/her ability to produce a convincing feigned death. So long as the Monk is the sole aggressor in sight, all nearby enemies will discontinue any spell mid-cast immediately once Feign Death is activated, convinced that the Monk truly has died.]
This makes life a little better for monks and notably better for raids that sometimes have to wait around on splits.
Third Ranger Tome: Something that makes your pet better or better yet makes SpotIV benefit groups/raids more.
Nevermore.
Third Shadowknight Tome: [Hogmart: 'Manual of Torment Mastery' - Increases the effects of all reverse damage shields cast by allies by x/x/x/x%]
I think this fits under "being useful without adding too much power".
Third Warrior Tome: [Ovisha: Gives an offensive stance to make warriors lower end DPS, tradeoff being not in s4 for life]
Should probably have a big ol' downside. Like Shaman Tome stance level downside.