Marvel is hiding DOFP from the Doomsday Watchlist intentionally
Marvel telling fans to watch X-Men and X2 before Avengers: Doomsday while leaving out X-Men: Days of Future Past is NOT a coincidence.
Days of Future Past is the movie that completely changes the X-Men timeline.
Wolverine travels back in time, stops the Sentinel program from fully developing, and creates a new future.
Now look at Doomsday.
Doom talks about people living “stolen lives,” while the returning X-Men come from a timeline that was fundamentally altered by the events of Days of Future Past.
The early Sentinel designs shown in Doomsday also closely resemble the Sentinels from Days of Future Past.
And this is exactly why Marvel isn't recommending it.
Because Days of Future Past would reveal TOO MUCH.
It immediately establishes that the X-Men timeline was changed, gives context to the returning characters, and could expose the connection between their altered reality and the events of Doomsday.
Marvel wants audiences to discover that connection inside the movie instead of being told about it beforehand.
That's also why the older Professor X, Magneto, Beast, Cyclops and the rest of the original X-Men are so important.
Days of Future Past isn't missing from the watchlist because it's irrelevant. It's missing because it's potentially the biggest spoiler. Marvel is hiding the movie that explains the entire mystery.