Bucky Barnes (odds of dying: 2 to 1)
Meet your most likely major casualty of the Battle of Wakanda. Even if Bucky (Sebastian Stan) somehow survives this massive confrontation, he is unlikely to make it out of Infinity War or Avengers 4 alive. If Steve dies (or has his powers taken away and looks more like his actual age), Sam Wilson feels more likely to pick up the shield than Bucky.
Bucky may not have been in control of himself as he committed unspeakable atrocities over the past several decades, but he remembers and feels guilt for every single one of them. Bucky feels the need to atone for what he’s done and will find the redemption he seeks by giving his life in service of Wakanda, The Avengers, or the entire universe.
Loki (odds of dying: 1 to 1)
Thor: Ragnarok pulled off a lot of neat tricks. One of the best among them was its ability to show just how interesting Thor can be even when Loki isn’t around. The Thor franchise can actually exist without Loki now, which is good news for the God of Thunder, but bad news for the God of Mischief.
Loki might (and I do mean might) succeed in saving his own life by surrendering the Tesseract to Thanos, but that won’t keep him safe forever. He will do what he needs to do in the moment, but we have seen Loki on a two steps forward, one step back path to redemption since Thor: The Dark World. There is no way Loki can make up for everything he’s done, but he will get as close as he can by eventually sacrificing himself to give his brother and the rest of The Avengers a chance at beating Thanos. But again, that may not even happen if Loki is already dead and I believe he will be.
Loki most likely dies moments after handing over the Tesseract.