AU where Thom survived Lioness Rampant:
He leaves Court for many years, back to Trebond and the City of the Gods to hide away and heal, build his magic up pure violet again. Even when he’s well again, he rarely stays more than a few nights in Corus - too many memories.
He does visit Pirate’s Swoop, though. Alanna is startled to hear him laugh with baby Liam (why name her firstborn after her brother when he’s already there?) She isn’t sure he’s laughed that freely since they were children themselves.
Alanna is hesitant, at first, about bring Arram - er, Numair - to Trebond, because no one knows better than her that it’s impossible to predict how mages will get along. They’re much worse than cats. At first her fears seem confirmed; both men are far too used to being the most powerful mage in the room, and it’s hard to imagine a tenser, more stilted conversation. Then Numair longingly mentions a rare book he wishes he’d never left in Carthak, and Thom’s eyes light up as he practically drags the other mage into what used to Lord Alan’s library, since much-expanded. Half an hour later, Alanna wanders off to find more food; neither man notices.
Over the years, Numair and Thom alternate between week-long magic!nerd sessions where they lock themselves in the library and occasionally remember to eat and months of haughtily refusing to speak to one another, generally over the disputation of some esoteric principle of transmogrification. There is no middle ground.
One time they both got drunk and changed the northern coastline
a little a lot enough that it took them three (sober) weeks to figure out how to reverse it.
Thom doesn’t like to get involved in the affairs of the kingdom. Of course he participates in wars - as if he’s going to let Alanna go into battle without backup! - but otherwise, he stays home. He never loses his drive to be famous though, to be the best, so he turns his magic to inventing. That lives on beyond his lifetime, and gives him to change to show off his cleverness. The globes at Carthak University that light when you clap aren’t his, but they give him the idea for automatic scrying mirrors, which can be used even the Giftless. Just breath on the surface and say the name of the person you wish to contact, and their own mirror (they have to have one with the same spell) will light up and grow warm to alert them. Smaller mirrors only do voice, not image, but suddenly Tortall has something that looks a lot like the modern cell phone.
George is the only one who works harder than Thom at finding Aly. They go to the Copper Isles together, and when Kyprioth turns up, Thom nearly blasts him.
I’ll just leave you with a mental image of the best thing we never got: Thom and Alanna back to back; her in golden armor, sword slicing through the surrounding enemies; him in a black robe, hands raised, firing amethyst lighting bolts; both surrounded by a nimbus of purple fire, matching orange hair almost ablaze with it.