This one seems like a long-shot, as it's hard to imagine the living embodiment of the concept of Death fitting in in the Marvel Cinematic Universe, but it has certainly been rumoured that "Lady Death" might appear. Thanos idolises her in the comic books - pretty much everything he does is solely to impress her - and the Infinity Gauntlet arc saw him wiping out half of the sentient life in the universe to make her happy. While we can't see how including her would work, we've seen talking raccoons, walking trees, gods, monsters, giant celestial beings and all kinds of weird stuff to date, so we won't completely rule it out just yet.
