There are two very real possibilities for a spin-off, both prequels. Maybe three. Let’s discuss…
A KNIGHT OF THE SEVEN KINGDOMS, AKA THE HEDGE KNIGHT SERIES AKA THE TALES OF DUNK & EGG
A Knight of the Seven Kingdoms is a novel that is composed of three previously released novellas (The Hedge Knight (1998), The Sworn Sword (2003), and The Mystery Knight (2010)), together for the first time. It is set nearly 100 years before the start of A Game of Thrones, and follows “Dunk” (future Lord Commander of the Kingsguard, Ser Duncan the Tall, who is also Brienne of Tarth’s ancestor) and “Egg” (the future king Aegon V. Targaryen, Maester Aemon’s brother). Bloodraven, aka Brynden Rivers, aka the 3-Eyed Crow, (known to viewers as the 3-Eyed Raven), has a cameo as well. It’s a wonderful book.
The other possibility is a spin-off leading up to, and covering, Robert’s Rebellion. It could begin with Lyanna’s betrothal to Robert; cover all events involving Rhaegar- from discovering a prophecy that would change his path, to his death at Robert’s hands at Ruby Ford during the Battle of the Trident; ending with Robert’s marriage to Cersei.
There’s a third, more expensive option. Expensive because it would require more dragon CGI than Game of Thrones already has- Aegon’s Conquest. It would cover the years during which Aegon I Targaryen, his sister-wives Visenya and Rhaenys, along with their dragons- Balerion, Meraxes, and Vhagar, conquered Westeros. It would cover the Wars of Conquest, as well as the first Dornish War and creation of the Kingsguard.
Any of these options would be amazing to see on screen. All three? Even better.