[Theory] How 'Days of Future Past' may directly cause Avengers: Doomsday
In X-Men: Days of Future Past, they used time travel to shut down the Sentinel program before it could really get started. At that early stage, the Sentinels looked very similar to the ones we see in the new Doomsday trailer.
Because of this, I think Doom's line about people living "stolen lives" is actually directed at the X-Men. By going back in time and changing their outcome, they essentially stole a future they weren't supposed to have. Also, think about the title of the movie. In DOFP, the exact day Mystique decided whether or not to shoot Trask was the ultimate turning point....it was the literal "Doomsday" for that universe. It feels like Doom is reviving those exact early-model Sentinels to bring that Doomsday back and collect on those stolen lives.
Since DOFP involved major time travel, that event could have easily caused an incursion that affected Doom in some way, like destroying his universe. This would give him a deeply personal motive. Plus, seeing the aged Professor X and Magneto is a huge hint that this is a direct continuation of that specific timeline.
I know Endgame also had time travel, and we've seen time-slipping with Loki and other movies. But the Fantastic Four have already been introduced to the MCU. Since this movie is introducing the X-Men for the first time, it makes a lot of sense for the core problem to originate in their universe before it becomes a multiverse-level threat that forces all the heroes to team up.
Basically, we can look at it like this:
The Main Antagonist: Doom, coming from the F4 universe.
The Catalyst: The X-Men caused an incursion that destroyed Doom's world.
The Climax: The conflict spills over into the main timeline, bringing everyone together.
Let me know what you guys think!