There's another founding member named Xiao who we know nothing about, and Mercy wasn't one of the founders. So maybe Xiao was involved in the medical stuff.
The Biotic Rifle is built using the same tech as Mercy's staff and 76's Biotic Field device.
Also, while there's plenty of evidence for Sombra being Ana Amari, I personally hope that's not the case, because it's just too similar to Reaper, 76, and Genji.
I'd rather Sombra be Ana's disciple, with Ana actually being dead. Then Sombra can have a rivalry with Pharah.
Taken to the melodramatic extreme, she could be the daughter of Jack and Ana, but Jack doesn't know he's a father. After Ana's death, Gabriel took the girl in and convinced her that Jack was responsible for her mother's death, so now she has a vendetta against him. Pharah is her half sister and resents her because she was Ana's protege.
That would be so much more interesting than "Sombra is a masked mercenary who is secretly one of the supposedly-dead founders of Overwatch and she's Phara's mother!"