Nebula (odds of dying: 7 to 1)
Nebula (Karen Gillan) is the most likely major character from the Guardians of the Galaxy franchise to meet her end. She is not an actual member of the team and Guardians of the Galaxy Vol. 2 makes it very clear that she only has one goal in mind: killing Thanos.
Drax may want revenge on Thanos, but Nebula has a much bigger beef with The Mad Titan. Remember that Drax originally saw Ronan, the guy who actually killed Drax’s family, as his biggest enemy before switching to Thanos after Ronan was killed. Thanos’ has been Nebula’s biggest enemy for her entire life.
Whether or not Nebula actually gets to strike the killing blow against Thanos, getting close enough to make the attempt might kill her. Yes, James Gunn loves Nebula and that might be enough to save her, but it would be a little odd for major Avengers characters to die while everyone from GotG makes it out alive. Nebula is not an actual member of the Guardians, so she is not guaranteed safe passage to Vol. 3.
Heimdall (odds of dying: 5 to 1)
Thanos is going to attack the Asgardian refugee ship and it is unlikely that we will only see anonymous Asgardians parish. This is probably going to be the first time we see Thanos kill a significant character with major historical ties to the MCU. We’ve seen a number of survivors already, including Thor, Banner, and Loki (Tom Hiddleston). Valkyrie and Korg probably make it out alive as well, so that only leaves Heimdall.
As Loki uses the Tesseract to help some heroes escape to Earth and warn The Avengers of an impending attack, Heimdall and Thor will probably be battling to give Loki as much time as they can. Thor will survive and be scooped up by the Guardians of the Galaxy, but Heimdall feels like the first hero to pay the ultimate price in order to save as many of his people as he can.