Marvel had a title for years called What if...? that had some amazing stories in it, and they did some one shots of it a year or two ago. They've also done some scattered alternate universe books like Squadron Supreme, Supreme Power, Ruins, The Last Avengers Story. There was an old series called Mutant X taht ran for 30 issues that was just fantastic
The MArvel equivalent of Kingdom Come is Earth X
There's also a series called Exiles, and the entire premise is that a group of heroes travel from dimension to dimension where things are always different and twisted and try to solve problems. Kind of like that show Sliders
Also, Marvel Zombies is just downright wicked