Beyond having a stealth ability, I sincerely doubt that she will play like the Spy. The cloak is really the only thing she probably will utilize from the Spy's kit.
Since there's only 6 unique heroes (especially in competitive where there's only one per team), the mask ability wouldn't work. The maps also don't allow very much room for her to change into that, either.
I also do not think that backstabbing would be a good thing to give her, since Overwatch is more fast-paced than TF2 overall, and oneshotting shouldn't be a thing, at least not for non-snipers. Furthermore, it would make her a very difficult character to balance - she'd be very ineffective in high levels of play but would be absolutely frustrating to go against in pugs, where she would stomp everyone. Even more so than Bastion. I don't think that's something Blizzard is striving for.
And a sapper would be kind of useless - Torbjörn's turret is not as impactful as an Engineer's sentry would be, Symmetra's turrets only have 1 HP anyway and Bastion has plenty of ways to counteract as well.