I think with her current beam, Sym will never be viable. For multiple reasons. Reasons, a lock-on could fix.
1: Tank Counter
Symmetra’s current beam is meant to counter tanks. However, there are many better heroes to break barriers or run through them. (Reaper, Torbjörn, Junkrat, Pharah, Tracer, Hanzo, Soldier, McCree and even Bastion)
If Symmetra had lock-on, she would be strong against DPS and there arent many heroes like that.
2: Improve her Bodyguard-Playstyle
When Symmetra is on a highground with Ana, her beam is just useless. It wont even really touch a dive DPS and Tanks have to much HP. Even if you manage to kill a D.Va, she has already killed Ana and its not a great trade.
However, a lock-on beam could really protect Ana. DPS like Genji or Doomfist wouldn’t have a chance and Tanks like D.Va might actually kill Ana but she would also charge up Sym and she would just run into the enemy team and slay everyone.
3: Improve her Offensive Playstyle (TP on Point)
Current Beam: Sym puts a teleporter on the point and (if you’re lucky) everyone goes with her. However, afterwards she is just useless. She is forced to fight in close range because her TP range is limited. In Close Range, Sym will just die. If she is lucky, she can get a turret up but even those cant really help. With a lock-on beam, she could acutally do something and her TP wouldn’t be a “Here I am, kill me”-Sign for the enemy anymore.
4: Balance the Meta
Right now, Genji, Hanzo and McCree are the strongest DPS (Doomfist excluded cause he gets nerfed) cause they lack counters. Symmetra 3.0 is weak against all of them. Symmetra 2.0 was decent against all of them. Her lock-on could make Hanzo’s and Genji’s life horrible and McCree would be forced to kill her with flashbang instead of a Support.
EDIT FOR FEB 2019: The lately popular snipers Widow and Ashe would be forced to play more defensive because Sym could TP on their Highground and microwave them.
So, without all the “Lock-on is unfair” stuff, a well balanced lock-on beam would improve Sym, make her rework a (little) sucess and change the Meta. I think it would be good to have lock-on back.
If I missed something or you think Sym’s primary could be changed in another way to get viable just tell me, I’m open for comments that arent “Lock-on was bad for the game” or “Learn to aim you noob”
EDIT FOR JAN 2020: Now, one year later, there are still the same problems for Sym. Buffs, Nerfs, Meta Changes…but in the end, Symmetra is still at the bottom. I’ll keep hoping she gets help someday! What do you think about her state and my ideas for her in 2020 Overwatch with Role Queue ectr? Still open for feedback!
EDIT FOR FEB 2021: Despite the minor health buff, nothing has changed for Symmetra in 2020. She is still terribe and her beam is a big part of the problem. Not much else to say ^^