I was bored on the ferry and imagining the things they could do to make Symmetra more relevant on not-defense.
In addition to my great "turret hat on a person's head" idea, I've also thought of:
1) Her shield can block 50 damage before breaking, but instead of being a straight bonus on the HP bar, it's a floating barrier that rotates around the target, leaving bits of them vulnerable at a time. This will increase granularity so they can adjust the strength of the shield without breaking Tracer and Genji utterly. Doesn't regenerate so she has to keep applying it (instead of the set-it-and-forget-it shield we have now)
2) She can deploy all of her turret charges at once and at range, like Reaper's TP but shorter. This will allow her to fight for points with her allies since her biggest obstacle is setup time. Preset patterns like Mei's Wall configuration can be cycled through with additional taps.
3) Main fire's range is increased and when used on an ally it projects a fading shield. Its not strictly healing and it can't reverse damage but it makes the primary fire more relevant than it is now. Perhaps the shield is porous like in the first idea.
4) Alternatively, main fire creates a cage around the target that blocks shots and is sustained by continuous firing, giving her some much needed survivability while chasing Tracers and Genjis.