"2020" (Earth-8410)
It is a dimension common to a number of beings. It was first seen in Machine Man II#1, but it soon came to be seen to be temporary home of Death's Head (Freelance Peacekeeping Agent), the home of Dr. Evelyn Necker (and thus Death's Head (Minion) and his prequels, sequels, etc.), and presumably of Wild Thing (Nikki Doyle). Unlike Earth-616 and its sliding timescale, the world of Earth 2020 seems to be focused around the specific era of 2020 AD, with the heroic era (equivalent to the "Modern Era" on Earth-616) having occurred @ 40 years in the past. Essentially, events in the MU occurred as they did on Earth-616 up to the stories published in the mid-1980s. After that, it's up for debate.
AIM (Advanced Idea Mechanics, Death's Head characters)
Ancient Wrecker (Midnight Wreckers)
Astro Vid Parlor (Broadani)
Athey (Dicemen)
Bain, Sunset (Baintronics)
Bainies (Baintronics)
Baintronics (Machine Man foes)
Baintronics Security (Baintronics)
Bones (Midnight Wreckers)
Brain (Midnight Wreckers)
Brickman, Miles (Machine Man foe)
Broadani (Machine Man foe)
C-28 Death-Dealer (Baintronics)
Cage (alternate Death's Head (FPA) ally) possibly @ 1992
Captain America (alternate Death's Head (FPA) ally) possibly @ 1992
Chance (Dicemen)
Charnel (Death's Head foe) - by Changeling
Death Metal (Marvel UK)
Death's Head (Freelance Peacekeeping Agent) - by Changeling & Snood
Death's Head (Minion)
Death Wreck (Marvel UK)
Dicemen (Death's Head (FPA)/Iron Man foes)
Dicemen Council (Dicemen)
Dr. Cyclobe (Baintronics)
Duffy (Death Wreck ally)
Hassle (Midnight Wreckers)
Human Torch (alternate Death's Head (FPA) ally) possibly @ 1992
Invisible Woman (alternate Death's Head (FPA) ally) possibly @ 1992
Iron Bots (Iron Man)
Iron Man (Arno Stark)
IU-104/E Eliminator (Baintronics)
Jocasta (Baintronics)
Machine Man (Midnight Wreckers)
Midnight Wreckers (Machine Man allies)
Mr. Fantastic (alternate Death's Head (FPA) ally) possibly @ 1992
Namor (alternate Death's Head (FPA) ally) possibly @ 1992
Necker, Dr. Evelyn Clarice (Death's Head character) - by Changeling & Snood
Sanctuary (Midnight Wreckers)
Slick (Midnight Wreckers)
Spider-Girl (Time's Arrow novel)
Swift (Midnight Wreckers)
Thing (alternate Death's Head (FPA) ally) possibly @ 1992
Vidiots (Broadani)
War Machine (alternate Death's Head (FPA) ally) possibly @ 1992
Wild Thing (Nikki Doyle, Marvel UK)