In a recent interview with World of Warcraft's Ion Hazzikostas, the developer states that cross-factions guilds are something that Blizzard wants to do, and they could be coming rather soon.
The interview, which was held by Twitch streamer Maximum (via PCGamesN), touches on the subject of cross-faction guilds, with Hazzikostas stating that it's something Blizzard are working on. According to the interview, Hazzikostas states it's definitely something the team is eager to do, but it would take a lot of "engineering problem solving."
"I would like us to do it before the next expansion, but it's in the hands of a lot of engineering problem solving."
The expansion Hazzikostas is talking about isn't the upcoming Dragonflight, though, rather it's more likely he's referring to the next expansion after that one (11.0). Still, cross faction guilds is a feature that many fans are interested in, even though there is a substantial Alliance vs Horde mentality still within the MMO's community.
Part of the process is already done, with cross faction grouping existing (and is awesome) as of a more recent Shadowlands update. And while the idea of allowing players to group up and play with each other, regardless of the faction they chose is an ideal Blizzard is working towards, it's still trying to walk the line that allows the faction rivalry that is a pillar of the WoW story to remain intact.
Hazzikostas in the interview also remains on the side that these features should be opt-in as well, including the possible upcoming guilds feature. This would allow players who want to more easily retain the feel of the classic WoW struggle between Horde and Alliance intact, as they can restrict instances, groups, and guilds to still be single faction entities (while the rest of us opt-in to play with whomever).
You can check out the full interview in the embed below, which touches on multiple subjects such as whether the Mythic lockout is outdated, class make up in raids, and much more.