Other subjects in the ASOIAF universe that GRRM wants to write about (Spoilers Extended)

In addition to numerous more Dunk & Egg novellas, and Fire & Blood II, I recently came across another SSM that lists some subjects that GRRM was at least interested in writing about about at some point:

To continue in the same view he has comprehensively ruled out ever doing a 'prequel' novel on Robert's Rebellion. Some ideas he has for future Westeros stories are (and again I stress he has not confirmed yet that he will do these) a novel on Aegon IV "The UnWorthy" from his POV and also a story set in Braavos and the Free Cities - possibly some kind of murder mystery type thing. -SSM, Interaction (Glasgow, UK): 4 Aug 2005

Writing something from the POV of Aegon IV (Unworthy) would be interesting to say the least:

With his father's death in 172 AC, Aegon, the Fourth of his Name, came at last to the throne that he had coveted as a boy. He had been comely in his youth, skilled with lance and sword, a man who loved to hunt and hawk and dance. He was the brightest prince at court in his generation and was admired for his wit. But he had one great flaw: he could not rule himself. His lusts, his gluttony, his desires—they all controlled him utterly. Seated upon the Iron Throne, his misrule began with small acts of pleasure, but in time his appetites knew no bounds, and his corruption led to acts that haunted the realm for generations. "Aenys was weak and Maegor was cruel," Kaeth writes, "and Aegon II was grasping, but no king before or after would practice so much willful misrule." -TWOIAF, The Targaryen Kings: Aegon IV

and a "murder mystery type thing" set in Braavos, which could possibly align with what he has so far with Mercy:

He said that LOVES to write about Arya's adventures in Braavos. He said he could write whole novels about.

TLDR: GRRM (as of 2005) wanted to do a story with Aegon the Unworthy as a POV and write a murder mystery set in Braavos.