In an interview at BlizzCon, Geoff Goodman, lead hero designer on Overwatch, revealed that there’s not currently any plans for future reworks in the game.
Whether you like Overwatch’s reworks or not, they’ve been influential in changing the metagame and overall health. Of course, some people say that to refer to Mercy’s weak state following her rework, others refer to Hanzo’s much improved gameplay. You can choose for yourself which this refers to.
But it appears that, following Torbjörn’s rework, the Overwatch team are happy to let the current roster sit still for a bit. When asked whether Blizzard will stop reworking heroes, Goodman said that he thinks that they will, eventually, even that he’s “hoping” they’ll get to that point.
“We don’t have any plans to rework anybody right now, but that doesn’t stop us if we decide that the game was changing in such a way that it is necessary again.”
Goodman did say that internal discussions have been had about a Bastion rework, though, saying they’ve “talked on and off” about his place in Overwatch. Given his specific niche and power, though, he said he doesn’t know if Bastion will ever get a rework.
You should be safe to main a hero without them getting drastically changed in the near future, at least.