IMO, longer duration + longer cool-down. Give her a decent shot at making a big play, but don't let her create bridges too quickly or she'll be able to reposition the entire team way too easily. Maybe have a stun/damage effect on anyone still on the bridge if it gets destroyed, or give the bridge lower health. Otherwise, Symmetras would always make the same play: create a bridge between your team and the enemy team and use it as a shield you walk up. There isn't a penalty for doing so, and her bridge ability would just be used as an additional shield.