What comes after Curse of Strahd? Well, before you can answer that you have to decide what happens at the climax of Curse of Strahd.
Assuming the party kills Strahd von Zarovich... do the mists clear and they return to whatever world or realm they were from previously? Or do the mists not clear and the party is still stuck in Ravenloft (whether that's the Baratok Valley that CoS is based in, or the entirety of the Ravenloft Domain of Dread that the 2E campaign setting was based upon)?
How you answer that question will influence which adventures come next.
If you go with the former-- that the party kills Strahd, the mists clear, and everyone goes home... you can then go into whichever modules or adventures you want, since they're now back in their home realm and mood no longer matters.
But, if you go with the latter... that you are keeping the PCs in Ravenloft even after what you'd consider the
conclusion of
Curse of Strahd... then you have some issues to deal with. Mainly that you've allowed the party to kill the Big Bad Evil Guy at around probably 8th to 10th level. Which means anything past that
if they remain stuck in Ravenloft is going to be very anticlimactic. They ONLY way to avoid that would be for the Barovia demesne to melt away upon Strahd's death, and the party then ends up in one of the other Ravenloft Campaign Setting demesnes and has to now deal with someone like Azalin in the demesne of Darkon or something.
However, if your intention was to keep the party in the Baratok Valley indefinitely, playing even more adventures and Ravenlofty modules past what is in the
Curse of Strahd book... then I'd recommend that you NOT END Curse of Strah at all. That you beef up Strahd von Zarovich so that he can't be killed by only 8th to 10th level characters. And you keep Strahd around as an all-powerful overseer of the Valley that doesn't getting involved with the party almost at all, and you compensate for that by putting in all of these additional modules or adventures into the Valley as other things for your PCs to do, and create several more lower-level BBEGs with their own plots outside of Strahd that the party can deal with.
If you really don't want to end the campaign in the Baratok Valley at 10th level and want to keep playing after that... in my opinion I would hold off on Strahd's death until what you feel is the correct end of the campaign. And remove Strahd as a potential antagonist until the party is much, much higher in level so that dealing with the Overlord of the land is in fact the climax. Heck, if I was running CoS this way, I'd probably not even have anyone in Barovia even know that Strahd even exists for the first 10 levels of the party. I'd probably keep the Castle's drawbridge up so no one could enter the Castle, never have anyone talk about having seen the lord of the Castle in decades, add a whole heap of additional modules like you wanted that had nothing to do with Strahd whatsoever so there's even more varied sandboxy stuff to do in the Valley, and even hold off on Madam Eva's card reading until the party reached like 12th level or higher. Only after the party was
that high in level would I have Strahd reveal himself to everyone in the Valley, and that's what the machinations of the PCs to destroy him would formally begin. And I'd beef Strahd up so that you'd need to be probably 15th+ level to possibly kill him. Because THEN you have a proper climax to this higher-level Curse of Strahd campaign.
But that's just me.
