I don't think Cates will kill Sentry off or anything that severe, but I do think Knull is going to have a reputation-building moment at Sentry's expense. As if killing first generation Celestials wasn't enough.
In an earlier post in another thread I indicated that I think someone from the Spider/Venom-verse should resolve the Knull conflict, albeit obviously with some help. This was in reference to someone proposing Thor and Thanos teaming up to deal with Knull. I'm opposed to that type of scenario.
Still, if Knull is as great as they lead us to believe, you need big guns to show up and play a role. The universe can't go to hell in a handbasket while they stand by idly.
In Blue Marvel's case, it just reaffirms that he's no Superman analogue, despite how some choose to interpret Kevin Grevioux's hypothetical premise behind the creation of Blue Marvel. Superman plays an integral role in just about every major DC event. They are definitely billing this as a major event, so I guess we can only believe that Adam Brashear fell asleep and slept while the tumult raged from the Negative Zone where Sentry presently sits across the known universe. Adam stuck Rip Van Winkling it again, even though his powerful mind, body and spirit doesn't require sleep at all.
But I'm getting ahead of myself. There's still an issue or two of the Dr. Doom series left. Who knows, maybe Adam gets depowered, killed off, or stranded in another divergent reality? Judo-tossed there by one of Doom's robotic scullery maids.