Congrats on the novel draft!!! I've come to chum the ask game waters with various off-the-wall Tortall next gen ideas, no pressure to pick up all of them:
Considering you've done so much with Neal, I'd be so curious to know if you'd ever be interested in writing a Jessamine fic? I'm not opposed to AUs where she goes for knighthood either instead of/with Neal, but I almost wonder if it would be more interesting to have her pursue more of a healer's path (Neal takes up his brothers' role, and Jessamine takes up Neal's). The gender play there is perhaps a bit more complicated, since I find magic to be, if anything, female-coded, but not by any means the 'proper' sort of femininity that a noblewoman is supposed to pursue! Especially cool I think if Jessamine doesn't just go the typical university route here.
My other thought is what you might do with any of the, well, sort of "textually missing but quasi-canonically existing" page boys, the sons of people like Gary/Cythera and Corram/Rispah, who really probably should have been at page training with Kel, but never appear on-page. On one hand, they're a blank slate for wholesome brotherly figures, but on the other, it could be cool to explore divergences between the next gen and their parents through them. Which is to say: what if they were bystanders to Kel's bullying? Or worse, what if Joren wasn't from a new fief, but rather Naxen or, irony of ironies, Trebond? The latter is so evil but I do like interplay between new nobles choosing progressivism vs. conservatism to shore up their shaky class standing... Your fics do class-through-marriage-politics it would be interesting to see how that stuff shakes out in other contexts.
Because I'm me, and have no shame about fishing for your lore, Elwene of Tirragen fic? How long can you use femininity to escape the shadow of your uncle before you trip head-first into the Delia of Eldorne trap?
Finally, Conté or Pirate's Swoop kid free space, but honestly I'm low key curious if you would ever write the (inevitable? tragic? legendary) Conté/Pirate's Swoop family romance arc and, if so, with whom? Hang on that's not a premise it's a prompt, so let's go rapid-fire: Lianne/Alan (bog standard love-doomed-by-the-necessity-of-political-marriage narrative), Vania/Thom (mage4mage? but maybe Thom's magical style is much more Conté-coded, builder of the realm, and Vania is a loose canon always trying to do dangerous great workings? you could also go lavender marriage here?), and for a real wildcard... Jasson/Alan (youngest son complex? named after the worst family member out of all our siblings which is saying something complex?).
This got SO long sorry!! Again no pressure to pick up on all of it :)
Tortall next gen messy business is SO fun!
I'd not thought of doing a Jessamine fic before--a healer's path makes sense for her, but I also very much like the thought of, mm, duchies are big and complicated to run, and Baird and now Neal have tangled themselves up with a LOT of other duties and seem to keep their spouses with them, which means probably younger siblings are going to get involved! So Jessamine needs a match with someone who is worthy of a duke's daughter but either isn't going to inherit his own title or doesn't mind splitting time, which is a fun tension in a society based around patrilineal inheritance. (See also part of the reason it's hard to match off Maura.)
If you peek, Jonthair of Trebond is is around a wee bit in one or two of my fics! And truly, I know Pierce wrote herself into corners, but WHERE are all these implied young men. I'm a bit too weenie to write any of them as villains in Kel's story but it really is sad that they aren't around the same way it's sad Maura's not around in Kel's books.
Elwene of Tirragen! I'm not sure what I would do with her (although her marrying Gary's invisible son would certainly be a hell of a thing) but really, I had her for one sentence in the manifesto and had to restrain myself from going down a rabbit hole.
And man, yeah, some Conte/Pirate's Swoop stuff would be fascinating but I've never quite found my way in on it! Like, I imagine Thom Jr. spent some time pining after Kally, but she'd be too smart to encourage that. Alan and Jasson is a VERY fun combination, though, of the options you threw out there I think that's where I'd be most likely to go. I have not introduced enough queerness to Tortall! (Says the woman, in all fairness, who wrote 70k of polyamory.)