They're never just going to straight adapt some Star Wars novel from the 90s.
First off because of continuity issues with the sequel trilogy.
Secondly because the novels basically all revolve around the cast of the original trilogy and there are serious practical limitations on that. It's also one of the fundamental weaknesses of these novels, they could never let these characters have much character growth except very occasionally after agreement by committee, and so they all feel static and boring having endless adventures that they always survive basically unscathed and unchanged by.
Thirdly because showrunners want to tell their own stories, not those of some franchise novelist from 30-some years ago. I understand the the Zahn novels are much more highly regarded than basically any of the other Star Wars novels, and having not read them since I was 12 I can't say if this is deserved or a function of them being the first major new stories and expansion of lore in the Star Wars universe in a long while when they were released, but still at the end of the day Disney and the people working for it aren't going to care enough about an old trilogy of franchise novels to directly adapt it. And I doubt Heir to the Empire really has diehard fans that would make that pay off (as opposed to people who have a positive impression from the 90s but don't particularly remember the books).
Fourthly if they just did a direct adaptation of the books they're probably have a complicated rights relationship with Bantam for adapting a book they published, or with author Timothy Zahn. I certainly don't know what rights contractual agreements arranged back in the day, but most likely Lucasfilm has much clearer rights to use elements from the novels (which they already have done) than to fully adapt them. Nothing some money couldn't solve in any case, but Disney would probably rather invest in IP they own lock, stock, and barrel.
Finally, when one of these rather expensive series is being released they want people to be online guessing what's going to happen next and getting hyped for the next episode to drop, not just saying "yeah, I read the book, X happens".
So yeah, maybe some more Heir to the Empire elements filter into Ahsoka season 2, or some sort of other series that continues the story from Ahsoka season 1, but I wouldn't expect much more than that.