It's not a twist at all. Everything has pointed in that direction from the very start. It's only not Garrosh 2.0 if the conflation to be made is that what made the evil Warchief story so abhorrent was that Garrosh cared about the orcs whereas Sylvanas cares about whatever gives the most negative karma in whatever situation she happens to be in. She's not understandable because we're not privy to her motivation at all and "lol, old god puppet" is the oldest motivation in the WoW book.
There'll be no turn to good and there shouldn't be. But Sylvanas being an old god puppet is just as much a retread of the Mists story, just with the one interesting bell and whistle of Garrosh subverting the old god to serve the goals of the orcs gone.