It's funny, because Mercy and Pharah have this weirdly rich dynamic that's kind of wasted in 'Overwatch' canon if you ask me. The whole 'I'm the guardian angel and you're my rocket-powered charge' thing is a solid foundation, but the real trick is to make it feel earned, not just assumed. I think a lot of writers fall into the trap of having Mercy be the soft healer and Pharah the stoic soldier, and that's fine, but it gets flat fast. For them to really click, you've got to find the friction. Maybe Mercy is secretly horrified by the collateral damage from a rocket barrage, or Pharah feels stifled by Mercy's constant, hovering concern—like she can't be seen as vulnerable. That conflict generates the actual chemistry, the push and pull that makes readers want them to finally get it together.
One tip I swear by is writing them outside of combat. A lot of fics just have them bantering mid-fight or patching each other up, and while that's fun, it's limiting. Throw them into a civilian context. Stuck in a safehouse during a storm, forced to share a tiny apartment on leave, dealing with bureaucratic nonsense at HQ—any situation where their armor and cadence come off. How does Fareeha handle a mundane problem without her Raptora suit? Does Angela get impatient or show a dry, sarcastic side when she's not 'on duty'? That's where you see the person underneath the role, and that's what makes a ship feel real, not just like two job titles flirting. My favorite fic ever had them trying to assemble IKEA furniture together, and the sheer frustration and eventual teamwork told me more about their relationship than a dozen epic battle scenes.