Blizzard naturally keeps it secret until Blizzcon, and so my predictions will be quite vague. This is what I think will happen:
Sylvanas serves as a binding character between expansions, much like Garrosh or Gul’dan. The expansion announcement cinematic will focus on her making a bold move and define the new threat.
Neither Bolvar nor Bwonsamdi’s “boss” are happy with her in BfA (Vol’jin questline), so I doubt she will ally with them. Helya’s pact in Legion and a hint that she’s still alive may show us the two working together.
Naturally Sylvanas is the major expansion villain, but shall we get to fight Bolvar too? It is uncertain how evil he will be.
The coolest possible thing would be Sylvanas defeating or deceiving the Lich King and claiming his crown for herself, thus gaining command over the Scourge. The logic here is that she’s disappointed in the free-willed Forsaken or free-willed Horde, and wants a mindless army to serve her goals.
I’d see this as the cool turning point before the final patch and raid, as Sylvanas will work for it throughout expansion. It may also be a shocking point during Blizzcon announcement cinematic – in that case we may even see “Lich Queen” in the expansion name :)
The outcome and the goal of the expansion is solving the undead threat problem once and for all, putting the mindless undead to rest and giving the free-willed undead hope and peace. Enter Calia Menethil as a new undead leader and a hope beacon.
Probably one of the things we may do throughout the expansion is liberating the Scourge under Calia’s guidance and making them free-willed! Finding a cure from dominated minds, and turning Northrend in a friendly undead state, how cool is that?
Shadowlands could be mentioned, a perk, a dungeon, or a world quest area like warfront. My idea is that we dive in it for some purpose, but, as a bleak mirror of the real world, it’s not the major playing ground. Colorful Northrend is.
The main location will be Northrend. We have a Taelia/Bolvar hook. Bolvar is not a hero anymore, he was quite menacing in DK campaign in Legion and during artifact weapon quests.
Vrykul presence in Northrend justifies Helya’s return.
Dragons find new strength already, and Wyrmrest temple is there. So I expect a huge subplot containing them. I do not expect minor local death entities like Bwonsamdi or Gorak Tul to play a huge role, although they may appear in a side plot.
New Death Knights and Level Squishing
New death knight models of every race means that there will be a plot of building a new undead army – either by Bolvar finally stepping forward or Sylvanas. Will it also mean reworked leveling questlines and leveling squishing? Because it will be very stupid to have a lightforged or a void elf doing Arthas’ biddings.
Looks good on paper :)