In many ways it feel that guilds are a secondary idea to the game, they aren’t really needed or that much useful anymore as a system. Even in Cata classic which is the expansion that brought perks and etc. they removed the need to level them or to be in them for some of the perks.
Many times the answer to player problems when pugging has been to tell them to join a guild but the game doesn’t really push you that way when you can do most content either easily solo or with a pug.
I think it would make sense to have some way to include a discord chat in-game as this is where most people nowadays hangout.
I would like some perks, bonus and levels to come back as personally this was the biggest reason I joined a guild when I wanted to start pushing content and it felt more natural to do so. Even if for many “cesspool guilds” like they called them were sometime not that great for me it was a starting point and I met some people from them and we made another guild to raid together.
I also really like the idea of a guild hall or guild garrison and maybe even have some kind of community events tied to them. Being able to see your guildmates more often when the only massive activity you do together is often raiding or just mythic raiding as this is the only content not really for pugs.
The cosmetic side is also not to forget, but I think just cosmetic banners is not enough to make me personally super interested they could easily had some type of bonus when using a banner like the rep ones.