You forgot an R in the thread title.
Sounds like Crimson Vow will be about the union of two vampire houses. I'm assuming Markov and Voldaren, partly because those are the most notable two and the only ones that could offer up marriage partners anybody can list of the top of their heads. It would present a pretty radical shift in power. House Falkenrath, and House Ithinktherewasnaotherone would be permanently sidelined and a united vampire faction would be free to dominate Innistrad.
I suspect Sorin might tie the know with Olivia- offering her dominance over the plane in exchange for agreeing to regulate the rate of human consumption. Or maybe it's Edgar who takes the vow and they play up the Edgar/Sorin conflict for drama.
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My guess is Olivia and Sorin, maybe as part of the deal for Sorin to get out of the rock.
Sorin already got out of the rock in time for War of the Spark. I don't think how he got out was ever described. It's possible he cut a deal with Olivia beforehand, but I suspect WOTC just kind of decided the "stuck in a rock" angle wasn't worth pursuing.
There's a lot less to go on with Midnight Hunt. Werewolves will play a role and apparently nights are getting longer and cults are catching on? Hard to say how significant any of that will be. They might push most of the story off to the second set and let Midnight Hunt serve as a welcome back to Innistrad.