They’re probably waiting (and hoping) for dive to die down in S10 with brigitte before buffing other tanks.
You could say the same with ana ,but considering they’re trying to kill the meta she was made for (triple tank meta) they’re probably looking for a way to buff her ,or maybe even rework her.
Seems logical. The forums will literally be on an argument war if jeff announce a rework on mercy too early. Best they announce it when they are ready to release it on PTR.
Messing with flat earthers aside, it’s most likely Ana, Bastion, or Reinhardt, which are the three characters that have suffered quite a bit from power creep.