It looks like a couple of major characters may be returning for Avengers: Infinity War. The massive Marvel event movie looks to be the culmination of everything that has happened in the Marvel Cinematic Universe up to this point. That being the case, pretty much every major character is going to show up, if only briefly. And, if Mark Ruffalo is to be believed, that may even extend to quite recent and presumably dead characters.
During a recent interview on Michael Rapaport's I am Rapaport podcast, the man responsible for bringing Hulk to life was asked which actors left "the best impression" on him after having worked with them. He then proceeded to name drop both Cate Blanchett, whom he worked with on Thor: Ragnarok and, more surprisingly, Tilda Swinton, who portrayed The Ancient One in Doctor Strange. That's of particular note because, as far as we can tell, there's no project outside of the MCU that the two have worked together on. And since Ruffalo recently worked on Avengers: Infinity War, it seems perfectly plausible that he worked with her on that movie.
This relies on connecting a few dots, but it does make sense. For one, Marvel likely wouldn't want to make Tilda Swinton's involvement in Avengers: Infinity War known because The Ancient One died a very meaningful death in Doctor Strange. That said, this also doesn't mean she's going to come back to life. The Sorcerer Supreme, aka Benedict Cumberbatch at this point, has access to different planes of existence. That being the case, he could somehow bring The Avengers into contact with The Ancient One to help them in their fight against Thanos.
As for Cate Blanchett's Hela? It's entirely possible Mark Ruffalo just meant he was working with her on Thor: Ragnarok. However, those familiar with the Infinity Gauntlet storyline from Marvel Comics will likely note why a character named Hela the Goddess of Death has a good reason to show up in Avengers: Infinity War. One of the driving forces for Thanos is his obsession and love for Lady Death. He's trying to impress her with a wave of death. Given Hela's fate in Thor: Ragnarok, it's entirely possible she's going to come back as the MCU version of Lady Death and become that motivation for Thanos in the movie.
Again, a lot of this is speculative, but there are enough breadcrumbs that make it seem like Hela and The Ancient One are both going to be in Avengers: Infinity War. Besides, Marvel wouldn't go out and get actors on the level of Cate Blanchett and Tilda Swinton and just use them once if there's opportunity for more. If you want to hear more with Mark Ruffalo, you can check out the full interview on the I am Rappaport Podcast. Avengers: Infinity War arrives in theaters on May 4, 2018.