IntellectualSetletI was also thinking about having a slow for fire, so if you get caught by pyro's flames you'd be fucked.This is similar to OW's Mei, who is very unfun to play against. Pyro with slowing in TF2 would be worse, especially because he can crit to kill with his secondaries and melees. Slows/stuns are not fun in FPSs, as we have seen with the Sandman and Natascha.
I'd argue the annoying part of Mei is the stun and instagib aspect of it, not the slow itself. I think it's hard to make pyro viable unless he is basically a better damage dealer than soldier unless you give cc elements to him (which airblast already is)