I’m yet to see the movie! Sounds exciting though!
Fun fact I actually did this in game, but never ended up RPing it lol. Sorta quit the game before getting a chance to play around with it.
Mainline Sarestha is a Forsaken, obviously. In short the backstory would be the same up to a point. She grew up in Lordaeron city in a fairly religious family (father was a priest, mother was a paladin) and largely enjoyed a very peaceful and happy life. She was about 13 on the day that Arthas returned from Northrend, killed his father, and unleashed the Scourge upon the capital.
In the main timeline her mother was killed that day, and her father, a pacifist who previously refused to use the Light combatively at all, was forced to destroy his wife’s reanimated body. This broke Sarestha’s father, and led him down a path of anger and extremism that eventually led him to the Lordaeron remnants that became the Scarlet Crusade. He took his young daughter with him, which led to her being rather indoctrinated with extremist ideologies, until her own death defending the Scarlet Enclave from the Death Knights of Acherus as an adult, some years later. (How she became a death knight, as it were).
In this alternate timeline for Alliance Sarestha, it was the other way around. Her father was killed, and her mother survived. Believing the rhetoric of Jaina Proudmoore, the family followed her to Kalimdor, alongside other Lordaeronian refugees. Thus, instead of going east to the Plaguelands, Sarestha went West, and ultimately came to be citizen of Theramore. Alas, her mother still dies in this timeline, but much later on, when Garrosh Hellscream unleashed his mana bomb upon the city.
Across both of these timelines, Sarestha’s strong desire to restore lost happiness remains, albeit on a level she doesn’t quite understand herself. On both sides she’s nationalistic, obsessed with Lordaeron restorationism (with either the Forsaken or the Alliance at the helm, depending, but the same core ideology). But in reality a lot of this comes from a subconscious yearning for the life she had before the Scourge - a life of peace and prosperity. This leads to something of a no-compromise attitude. Mainline Sarestha doesn’t despise the Alliance, at least not anymore, but she regards them with great suspicion, fearing that they plan to reclaim Lordaeron at any moment. As far as she’s concerned, the Forsaken are the rightful heirs of Lordaeron and the Stormwind-led Alliance has no claim whatsoever. Alliance Sarestha actually does hold the Horde in contempt - the destruction of Theramore and Teldrassil makes them as irredeemable in the Scourge, in her own opinion. Unsurprisingly her attitude to Lordaeron (and Dustwallow Marsh, even) is total Alliance control, believing that the Horde do not deserve these lands.
Anyway I’ve really only thought about those two particular possibilities. I’m sure there’s a universe where Sarestha remained Scourge, where she remained loyal to Sylvanas, and others… but I haven’t thought about them quite so much.