Mercy and Moira need more interactions.

When Moira was first revealed, she was portrayed as a kind of "Evil Mercy", and this shows in their designs and backstories. Mercy is a caring person who helps others not for glory or curiosity, but because it's the right thing to do. She's willing to put up with a lot of stuff to protect others, even if it causes her frustration. And she has a strict adherence to ethics (she scolds Ana for turning her healing technology into a weapon, for example).

Moira, on the other hand, is noteworthy for her complete lack of ethics. She's driven by an insatiable curiosity to learn more about life and the world, and while nobody will deny her discoveries, her reputation has taken a real hit. Even her default skin is based on Mercy's, just primarily black and with ominous robe like extensions. And who could forget that demon looking suit she was wearing in her origins trailer when she was turning Reyes into Reaper (please add that as a skin in the next Archives/Anniversary event Blizz)

And yet, despite all of that, there's very little interaction between the two. They have a pre-battle line where Mercy insults Moira for her lack of ethics, and Moira retorts by bragging about her discoveries, and Moira says "our guardian angel" sarcastically if she kills Mercy, but that's about it. In fact, when Moira was released, she and Mercy were given sigil sprays that could be lined up to show them standing back-to-back, possibly showing that they parallel one another as rivals/enemies, but nothing really came from that.

So I'm hoping that, when Blizzard starts making progress again with the story, they expand more on Mercy and Moira's history. I like to imagine it as Mercy is normally very kind and patient, but Moira is one of the few people who can legitimately piss her off, while Moira has a massive grudge against Mercy for shutting down her work, despite the progress and discoveries she made. (It's never been revealed who shut down her work, but it being Mercy would make too much sense. Correct me if I'm wrong, though.) They could also extend this rivalry in game, with more pre-battle interactions of the two bickering (and maybe even having the rare sight of Mercy being legitimately mad at someone), and more companion sprays between the two showing them at odds with one another, like what Hanzo and Genji have.

And there's potential in the story, too. Moira is confirmed to be working at Oasis University following the downfall of Overwatch, and in Recall, Mercy was shown to be in Iraq, where Oasis is located. So they could easily make a comic where Mercy runs into Moira at Oasis, sees what she's doing, and decides to accept Winston's message and rejoin Overwatch (or something like that).