I'd rather have a "Payday" game mode where you have to extract the payload/or whatever and defeat Guards/Black Talon ops in a 4-6 player co-op.
If anything, the Bastion short showed us that there are thousands of Bastions and several Reinhardts out there, so we aren't "Superman" and invinicble just because we are in Overwatch.
Add Reinhardt-ish tank enemies, Bastion/Robot/Synthetic enemies, 08/15 human grunts and some snipers and you have a good set of enemies that can make a gamemode like this a lot of fun from time to time.