honestly, the ship dynamics are way more varied than just the obvious one. Obviously, Mulan/Shang dominates everything—it's the canon endgame from the movie, so it gets a massive amount of fics, from fluffy post-war reunions to angsty 'what if Shang figured it out earlier' scenarios. That's the bedrock of the fandom.
But the real interesting stuff happens in the non-canon pairings. Mulan/Shan Yu has this huge, fiery, enemies-to-lovers pull that I see constantly on AO3. It's not for everyone, but the tension between 'you dishonored my army' and 'you're the only worthy opponent' creates such a dark, compelling dynamic. There's a ton of AU fics exploring that. Then you have Mulan/Yao, Ling, or Chien-Po, which often come up in poly or 'what if she fell for one of her brothers-in-arms' stories, though they tend to be less common.
I've also seen a surprising amount of traction for Mulan/Khan, her horse. It sounds weird out of context, but it's usually handled as a deep, mystical bond, sometimes with shapeshifting elements, tapping into that 'loyal companion as soulmate' trope. It's niche, but dedicated. And crossover ships pop up, like Mulan/Merida or Mulan/Jasmine, where fans mash up the 'warrior princess' archetypes. The fandom's old enough now that people just love to play in the sandbox, moving far beyond the original film's framework.
The popularity really depends on the platform too. Tumblr and Discord servers might be buzzing with Mulan/Shan Yu edits and headcanons, while a site like FanFiction.net might have a higher concentration of classic Mulan/Shang family fics. You can tell what's hot by the tag counts and the kudos on recent works. Lately, I've noticed more fics exploring Mulan's relationships with other female characters from the lore, even original ones, which feels like a natural expansion. People just connect with her strength and want to see her in every kind of love story imaginable, I guess.