What about a +2 armor upgrade on the TechLab for BCs and Vikings?
Though I think nerfing Parasitic Bomb would go a long way to improving the Viking’s viability in TvZ. Reducing the damage of PB from 120 to 60 would mean that the Vikings would have half their HP left, as opposed to 15 HP left after a PB… meaning that the Terran player has a meaningful choice to either run or fight it out in TvZ air battles. TvP Air battles still seem to favor the Protoss, but it’s not as grossly unmanageable as the TvZ imbalance.
Making Corruptors more expensive Supply wise would force the Zerg players to actually make meaningful tech choices, instead of having a half dozen Corruptors, Vipers, and Infestors which they are going to have almost no matter what else they have in their army.