Wolverine: suicide. 200 years and 15 lovers in the future, Wolverine has reached the end of his rope. He's lost too many. Jubilee, Kitty, Doreen, Maggie, and others too countless to name. some were like daughters, others like lovers. He's always felt he was the outsider, but they had kept him sane. And so, one drunken night he puts his claws through his eyes and scrambles his own brains.
Cap: I definitely like the idea of him passing on the mantel to someone he is proud to have worked with. I'm thinking Peter Parker, but given the approximate biological age, I don't think so. Perhaps, some point in the future, Patriot or Victor Mancha. Also, cap should die from cancer or something caused by the Super Soldier Serum that he doesn't tell anyone about.
Hulk: I want him to literally have his head ripped off by Molly Hayes... that's all I'm saying about that.
Spider-Man: Killed in battle as an Avenger. They're fighting someone like Zemo or Hydra or something and it seems absolutely hopeless, but Peter is more determined than ever. The reason: Hours earlier MJ told him she was pregnant.