Swat the Mosquito
One of the joys of Overwatch is how supremely balanced the entire experience is for the player. No matter which character is chosen there is another one in the roster that can neutralize them. Pharah and Reinhardt might not be the pair that pops to mind the most when thinking of perfectly opposing Overwatch pairs. Pharah is a high-flying missile machine and Reinhardt is the definition of a tank, all that he is missing is a large gun coming out of his chest. This difference in altitude (and everything else) is precisely why they would make for a great and unique game mode.
Imagine a team full of Pharahs going up against a team full of Reinhardts, or just a 1v1 duel. The Pharahs would, obviously, fly high above the Reinhardts raining “it” from above. Meanwhile the Reinhardts would be on the ground using their shields to fend of the Pharahs’ barrages and trying to use their hammers to swat the flying menaces for the sky. There is a danger of things getting frustrating very quickly and certainly isn’t a mode for those who play Overwatch competitively. Still the mayhem could make for some wild and literally explosive matches.