I've heard S6's theme is "Family." With that in mind, I feel like Samantha is probably dead or in a coma, and Oliver has to take care of William for most or all of S6. They really can't go anywhere else with his "inner darkness" storyline; this season kind of wrapped all that up, I feel. So how else do you create drama in Oliver's life? Make him figure out a way to juggle being a mayor, Green Arrow, and a father. It will also allow for lots of bonding between him, Diggle, Rene, and Quentin as they all struggle with different fatherhood issues (Diggle's apparent marital issues with Lyla perhaps getting in the way of his relationship with JJ, Rene trying to be a father again, and Quentin struggling with Black Siren's existence).
Plus it adds a dynamic to the show that has been missing for a while now: normalcy. Since Moira died, there's not really been a sense of "real life" to this show. It's all about vigilantism. S5 went toward fixing that with a lot of Oliver's time being taken as mayor. We saw Quentin, Thea, and Rene all working with him. But I'd love to see some stuff outside the mayor's office and the Arrowcave. Can we see where Oliver's been living? Maybe start using the homes (Felicity's apartment, Diggle's house, etc.) as set pieces more often?
Anyway, I doubt anyone else will die. Boomerang, maybe. But nobody else. Unless Talia's actress is unavailable, I'm sure they'll bring her back. They love Nyssa too much to kill her. Thea, Diggle, and Felicity are all safe. We know Rene and Dinah are series regulars now. Curtis is safe (actor just tweeted an image of him and Black Siren with #SeasonSix). Quentin's safe–because Black Siren's safe, and they'll use her relationship with him next season. Slade is hopefully safe, but if anyone else dies, sadly it'll probably be him. Evelyn, it's a toss-up. I can see her dying so they can sweep that situation under the rug, but I can also see her surviving and Oliver trying to work on redeeming her. So, we'll see.