Yea, that would be really cool. Love the chain heal idea that Shamans have in WoW. Actually, there are a lot of really good healer ideas in WoW that could be translated into Overwatch.
A common suggestion I see for Mercy is the Pally Lay on Hands ability which is essentially a super large heal (almost max health) with a really long cooldown instead of Rez.
Actually… I came up with some Mercy changes that I kinda stole from WoW too: