Okay, so, I've been thinking about it because it keeps coming up, and here's how I'd try and fix Symm without outright discarding and replacing pieces of her kit:
M1 & M2 can stay the same. Maybe a little bit longer range on M1, and a little bit faster move time on M2 (or tie speed to ball size), but honestly, both are pretty great.
Turrets: Increase count to 8, give her the option of placing either damage/slowdown or healing turrets. All turrets available at the start of the game, once game starts cap stored turrets at 4-6. Dramatically reduce turret setup time. Slight stealth effect on turrets, not outright invisibility but... reduced visibility.
Shields: change from flat rate to fixed %. Base it on lowest health character. Set it to maybe 1/6th of character's health. Add the ability to boost shields temporarily, Lucio style, but on a per-character basis. Maybe an additional percentage, maybe a fixed quantity. Cooldown for boosting should be per character. This adds an interesting dynamic, as you're forced to both choose which characters to boost, and when to boost them. Is it better to fortify your tank at the start of the fight, or do you wait until they're at lower health?
Teleporter: This one is kind of tough to wrangle. Right now, ults are 100% team fight oriented. Any ability that doesn't contribute to the team fight is basically worthless. So, to address that, I'd have her teleporter apply a short-term buff to anybody who comes through it. That should be in terms of an extra layer of shields (that decays much more slowly than the fortify shields) and maybe a damage boost, though if it does get one it should be significantly lower than Ana's. Teleporter should also have DRAMATICALLY reduced visibility and audibility. I would propose that, after placing, it should slowly fade from view as it goes unused, eventually becoming nearly invisible and wholly inaudible. When someone goes through it, it surges back to visibility, then starts fading again. This makes *when* you place the teleporter important, since you need to be away from view. Charges stay at 6, at least to start with, and you should be able to build up 50% charge while the teleporter is out. Numbers involved here are all very flexible, you'd clearly need to figure that out through trial and error.
Addition: give her something to codify the "bunny hop around and melt people" tactic. I'm thinking some kind of movement ability. You could justify a double jump through hard light stuff, but I'm kind of inclined to tie into the reduced visibility stuff somehow. Maybe have her vanish for a moment at the top of a jump? Just to mess with people.