Another small change Mei could have

She can maintain her ice wall while she uses primary fire on it, including keeping each pillars health full. Helpful for those cases where you wall off a Bastion in turret or a Roadhog who is ulting.

Essentially the wall lasts for the normal time, and during the normal time any primary fire on it keeps the HP maintained at whatever it is, and any time it's not used the HP decreases with damage.

Then, if primary fire is being used on the wall when it would normally expire it instead sticks around until primary fire is no longer applied to it.

So this way a Mei cannot wall forever, but by sacrificing some mobility and offense capability (she has to stay near and looking at the wall) she can keep it up for a bit longer.

Just a thought, mostly because i've been chewed by a few roadhogs recently :P