It was more than obvious that someone had to die in the previous Avengers movie - and since it couldn't be any of the main characters, and since Scarlet Witch was way more powerful, that someone had to be Quicksilver. Set to be an emotional turning point for Wanda Maximoff, Quicksilver's death might have been a bit disappointing - the fastest on the team being killed with bullets is kind of ironic.
Nonetheless, as it appears, what Joss Whedon wanted to be a definitive death could be changed. According to Screen Rant (citing 4chan leaks), it could happen as soon as the Infinity War arrives. And there are quite a few reasons why this should happen: Besides the fact that Quicksilver is a character with a rich history in the comics, Marvel and Disney should make the most of this opportunity, since most of the established actors from the shared universe will reach the end of their contracts at the end of Phase III.
And, besides small cameos via flashbacks or visions, there could be two ways to make this happen: Resurrection and time travel. And either is possible, since until now the entire MCU has been built upon the quest for incredibly powerful artifacts, some of which could alter time, resurrect the dead, or even change reality entirely.
Doctor Strange could be the one to bring Quicksilver back, using the Eye of Agamotto - however, changing time like that could create quite a few continuity problems (and we all know how that is, thanks to the X-Men shared universe).
But Quicksilver could be brought back via the Soul Stone, which hasn't been found yet. This could avoid all of the problems time travel brings.
And then there is another possibility: What if Scarlet Witch realizes her potential and brings him back herself? After all, her powers are limitless at this point (or, at least, we believe they are, since she was even able to subdue Vision).
So what do you think? Should Quicksilver return? Or should Marvel focus on other characters for the MCU's Phase IV?