Thad getting a redemption arc would be so great but here's my funniest reasoning for it, Barry would finally have one blonde descendant. He would definitely eat that up. Like each grandchild is special: Jenni is his only granddaughter, Bart is literally named after Peepaw Allen, and then there's Thad the little teeny tiny feral cat who j needs a hug and he's got all this blonde hair and Barry's like yup he takes after me. But the funny thing is they all take after him in one way or another.
The concept of Thad getting really into "kid interests" is something I'd love to see more of in redemption fics (not knocking any fics bc I love people who write redemption fics for Thad). Like I wanna see Thad get super into puzzles and maybe collectibles like Yu-Gi-Oh! cards or Bakugan toys or train sets.
Especially if it's something Bart would have no interest in. I like the whole "mirror twins" thing between the two of them.
I wanna write dad!Don Allen so bad. Don listening to Bart talk about the latest video game he's into and Don's like, "I don't know what a Samus is but I like that you're happy." Don telling Bart that he's so much like his mother. I'd love to see him take Bart to skate parks and cheer practice and teach him tricks on how to sleep through the night. I also think Don would be impressed with all the things Bart's been through and how much he's learned in such a short time.
Also him being a dad to Thad and being the one to really understand him and how he feels. Taking a lot of personal time out with Thad to help Thad build an identity of his own outside of being Bart's clone or "the Thawne's last hope for destroying the Allen's". I could even see Don seeing a lot of himself in Thad and relating to his struggle for identity and attention and affection. It's hard enough being a twin let alone a clone whose purpose was already written out for thousands of years. Don breaking through the Thad's walls and proving his love for him as a father. Reminding Thad he isn't the enemy constantly.
Don being in a family full of people with big feelings and being so happy to see Thad come out of his shell in more positive ways. Don laughing at Meloni and Bart's antics while also being so happy to see how close they all are. Don and Meloni being up for hours at night talking about how their family's come together and how amazing it is to see Bart and Thad grow up.
Don's smile at the end of the day knowing that he wouldn't wanna be anywhere else. There's sm I would love to do with Don as a father and husband. He was robbed of all of that fr.