The Sentry/Void is my guess for "The Thing Behind the Door". And no matter what his powers, bet Doom could take him down quickly, and probably absorb his power to boot...(after all,he's done it to the Surfer...)
Wait.
Um.
Hmmm.
Doom with Sentry-powers.
What would you call that? Darker Reign? Brighter Reign? Could Doom be cooperating with Osborun just so he can...