TES Online does not take place in TamrielIt takes place in Coldharbour, which has already been said to be a cruel mirror of the real Tamriel, your characters and others just think they're in Dawn's Beauty rather than Molag Bal's Arena. It handily explains the oddities with the set-up, explains away the problems with the handwave justification for being able to come back and is perfectly in the Corruptor's perview.
The Bigger good is going to be The EbonarmOr at least there will be a reference to him, Sai or the Citadel.
The plot is another in-universe bookPutting together a coherent timeline of Tamriel is already difficult, thanks to the flexible standards of scholarship found in the books scattered through the games. If this game proves unpopular or is rife with continuity issues, Bethesda will simply declare it to be an in-universe work and therefore of debatable historical truth.
Lord Harkon will appearHe's a devout worshiper of Molag Bal and an immortal vampire. The timeline is also right for Serana to make a cameo, assuming the guess of her imprisonment being second era is correct.
If the game looks like it's going to fail, Zenimax will introduce Tiber Septim himselfIt would be a good way to draw more interest and possibly save the game. And if it doesn't work, they can instead end the game with Septim taking over.
Sheogorath will have a prominent role or at least more than a token appearanceThree reasons. First, with Molag Bal having a prominent role, more of the Princes are bound to be included; Second, he's pretty much the Ensemble Darkhorse of said Daedric Princes; and Third,cheese!
Boethiah and Mehrunes Dagon will be prominent Enemy Mine allies of the player characters.Boethiah hates Molag, and Dagon's driving motivation is the destruction of Nirn- he's not going to stand by and let Molag do it.