Caustic's gas aren't even that good.
First, it takes time for his gas traps to activate and enemies are usually a good distance away, when the traps can start to take any effect on them. So most of the time, his traps are just used as a *-off-sign or to warn you (Only the one playing Caustic) that there's someone near the trap.
The only situation i find his ultimate good, is when you want to cover yourself or an knocked teammate and make your enemies stay out of the way. (Until they find cover a second after)
So i still think he's a bit weak compared to what some other legends's ultimate can do: Artillery strike, mortar strike, give yourself a (Kind of) thermal vision (Bloodhound), call in a Care Package that's almost just as good as a normal airdrop, and more.
However:
*Re-work Caustics passive to allow his team to see through the gas if within a 'x' foot radius of Caustic. Basically, Caustic can provide them 'gas masks' within a certain radius (thinking a small radius, nothing too large)
Is not a bad idea.