Want to make Caustic relevant? Make his gas prevent gunfire
Proposal: if your Legend is caught in the gas, they're wheezing/malfunctioning too much to fire either weapon they're carrying.
I would rather that Legends that end up in his gas are unable to fire because they're too busy coughing, than lose health. I want both. But I know that something losing health is the whole point of "Caustic" since the word means eating away at something.
After his deployables became destroyable, his viability worsened. Being able to cordon off doors, cut off escape routes and punish players for entering a space he is controlling was greatly reduced. It's not to say that I don't agree with it. But at the same time, if a Legend gets caught in the gas, they should have to do more than just spray to stop the trap from going off. If a Legend gets out of the gas cloud, then they can destroy the gasbag, or someone on their squad can kill it while their teammate is still in the gas cloud.
Combo plays as a result of this:
Gibby and Caustic: Create a safe space on the battlefield with a dome and gasbag. Toss up the dome, shoot a gasbag, heal up accordingly.
Horizon and Caustic: Toss two ultimates into the same space, and now Legends cannot shoot the gravity well.