Barbare
I would recommend you to be a little cruel in the story at the beginning of the story in the sense that as the saying goes "a life for a life". I mean, Edmure is not born in exchange for William being born and Hoster for Minisa, so he is the heir and Hoster does not touch the balls. It is cruel, but it is Westeros. Now, with how young William is, it would be interesting that with him Over time something similar to the Tarbeck and Reyne happens but with the Freys and the House of Bracken, but when William is older and has shown that the Tullys, like the Lannisters, are ruthless and earn the respect of the great houses since the Tullys are always underestimated, as soon as the William marriage is more ruthless when suffering a war, he could decide marriages different from the original story, that is if respecting his cousins, he with Janna Tyriell or his sister, Lyanna Stark , Cersei or Elia or Lynesse Hightower too and I know it's crazy but in exchange for some Rhaella Targaryan Valyrian steel swords, it would be fun. Besides rebuilding Harrenhal it would be scarier for everyone since all great houses have one superfortress because he gets the best.