I had a thought the other day while seeing her shield ability video on her character page. Look at the way the shield pops around Hanzo at first, then gets absorbed into his armor. So, here's my thought:

Make Symmetra's shield function exactly like Zarya's projected barrier (200hp, 2 second duration, same cool down, her signature blue instead of pink), except whatever damage it doesn't block becomes armor for whoever it was projected on.

Throw it on Lucio, who takes 100 points of damage while shielded, Lucio now gets 100 armor after the 2 second duration of the shield is over. This allows Symmetra to be far more engaged in the fight versus placing shields on players as they spawn. She can play reactively and help protect her team (she would also combo well with Zarya).

My limitations for it would be simple: If the teammate that the shield is projected on takes no damage in those 2 seconds, they only get 50 armor. This prevents Symmetra from making everyone tanks before the fight and also gives her the incentive to use it during a skirmish versus a standstill. The other limitation would be the same as the current one: Once an enemy has armor, Symmetra can no longer shield them until they once again have no shield. This WOULD include combat however, so if an already shielded ally dips below their current shield amount, they become a viable target again(the new shield would replace the old one, not stack). This would allow Symmetra to become a defensive protector, while not infringing on Zarya's role. Plus it would give her shield ability much more utility, as it's pretty lackluster at the moment.

A third limitation that might be necessary would either be a shield cap after the initial barrier dissipates(maybe 100 shield max?), or potentially making it armor that doesn't regenerate like Torbjorn's.