Now, the title does sound like some weird "twincest" type of thing -- like what Quicksilver and the Scarlet Witch have in the Ultimate Comics -- but it is not. As many Marvel Comics fans know, Pietro and Wanda Maximoff are fraternal twins who were orphaned at a young age. And because they had no parental figures growing up, they developed strong loyalty between each other. And it is because of the loyalty they share that Quicksilver may come back.
Any Marvel Comics fan would know that the Maximoff twins are inseparable. Wherever one goes, the other follows regardless of what may lie ahead. What I'm getting at is that their loyalty can and, potentially, will lead to the revival of Quicksilver.
Although 20th Century Fox intentionally casted Quicksilver in "Days of Future Past" to spite Marvel and writer/director Joss Whedon, both characters are of different origin: one is a silver-haired, angsty, mutant teenager who uses his power to steal Twinkies and Atari video games, born and raised outside of Washington, D.C.; the other is of Sokovian descent who was genetically enhanced to run at super speed with the power of Loki's Scepter, and has a much favorable white-silver hair color.
When it comes to being cannon to the comics, Aaron Taylor-Johnson (Pietro Maximoff/Quicksilver) is closest. Although Fox had the courtesy to reference that Quicksilver is Magneto's son, Aaron Taylor-Johnson is the best Quicksilver.
Another point to review is that there are numerous dead-but-not-dead moments in the MCU. Agent Phil Coulson, Loki, Nick Fury, Bucky Barnes (Winter Soldier), and, I am greatly positive, Quicksilver. So there is no definite guarantee that Quicksilver is actually dead.
And if he is dead, there is one way he can come back to life: his sister, Wanda. In the "House of M" story line, the Avengers and X-Men regain their memories and attack the House of M. Upon having his own memories revealed to him, Magneto attacks Quicksilver, killing him in a rage. The Scarlet Witch, without hesitation, runs to her brother's side and revives him.
So, if 20th Century Fox thinks that Marvel can't use THEIR characters, they are in the wrong. Quicksilver may very well come back. And if "X-Men: Apocalypse" tanks, like how 2015's "Fant4stic" "Tranked" -- pun intended -- then Marvel will eventually have the rights to the X-Men and Fantastic Four returned to them.
Keep in mind that this is NOT confirmed, but to the true Marvel Comics fans, the ones that despise the X-Men movies (or the MCU movies), there is that one gleam of hope that has white-silver hair.