The best way to solve Symmetra is to change her shield ability and give her ultimate a toggle between 2.
Change her shield ability to an ability that releases a massive pulse that slows down the enemy team temporarily (3 seconds for example) or if you want to make her a healer, make it a pulse that heals 50 HP immediately for everyone around it or something like that.
And for her ultimate, it should be able to toggle between the teleporter and a new ultimate that's essentially an AOE effect that slowly deteriorates the opponents health and maybe slowly heals your team. Symmetra remains invunerable like Zenyatta during his ult.
Those are a couple of changes off the top of my head that don't seem too OP and would make her a lot more interesting, versatile and most importantly, proactive in a fight!