I haven't seen anything in the manga regarding Gyokko having a backstory though I do agree that having one would be great.
Personally, a good chunk of what I'd been curious about did get addressed in the spin-offs and in Giyu Gaiden (namely, how Inosuke learned to talk when raised by boars or what Giyu did in the time between meeting Tanjiro and until the latter finally became a Demon Slayer) which I'm thankful for. I definitely want more Gaidens, mainly because the current ones are awesome and I'd definitely recommend them for people to read but also because the artstyle (by the Kimetsu no Aima artist) is gorgeous and I wanna hear about how the other Pillars went about their duties during "normal" times.
Here's something I think might get addressed soon but if I recall correctly at the end of Chapter 134 Muzan refers to Nezuko as "Nezuko Ubuyashiki" and that really grabbed my attention and I eagerly wanna know what that was about.
For something that's less likely to happen, I wanna see how Genya discovered his ability to eat demons and gain their powers and I'd very much wanna see an arc where we follow the Kakushis more, like we got an arc for the Swordsmiths, though admittedly they probably wouldn't have the potential for an entire arc.