During one of Doom's monologuing classes, Latveria is under siege by the Four Horseman of Unicron. They destroy most of Lateria but leaves some parts Doomstadt intact. They deliver a message to Doom. Due to a distortion in space and time, Unicron (his physical form) has been transported to the Marvel universe. He wishes to return home but decides to have a little snack before he leaves and sets his sites on Earth. He senses that Doom is the only immediate threat and chooses to cripple him by setting his country ablaze. The Heralds state that Unicron we arrive in 1 weeks time to consume him along with Earth.
Round 1: Setting in near Saturn. Unicron's Horsemen and former Heralds vs Doom-Bots and whoever Doom has partnered with in the past
Round 2: Setting is by Earth's moon: Unicron (fully Transformed and hovering towards Earth's atmosphere) vs Whatever Doom has at his disposal.
The rules of engagement are simple:
1 year prep before fight starts
No time travel
BFR is off
Bloodlust in on
EDIT: Switch it up a bit to give Doom some leeway.