I wrote
a thing for DMsGuild as well, and it had a section on changing up different plot points. The salient point being that I too felt like "restore hope" should've been a bigger deal, and also perhaps Sergei's role maybe being a bit more obvious (assuming the PCs do their homework at least). But yeah, that third gem in the Wizard of Wines winery was a big key, and considering how the wines are referred to as one of the "last escapes" or "hopes" of the Barovians (until the winery goes dark and the dream pastries show up), it feels right making a bigger deal of that.
The part from my DMsGuild release that addresses this:
"The Lost Gem
The first missing gem from the Wizard of Wines Winery was stolen ten years ago and its fate remains unknown. Meanwhile, wine is pretty much the only thing Barovians look forward to -- other than Morgantha's dream pastries, which are made from crushed up children, so that's not really a positive thing -- and it's in short supply. As it stands, no matter what the players do to save the Wizard of Wines Winery, the truth is that the business is doomed and Barovia loses all hope of ever tasting wine again. While perhaps that is fitting in a cosmic horror campaign, Ravenloft is grounded in gothic horror...and what's a more iconic gothic horror drink than wine?! (Nothing, that's what.)
In the spirit of vintners and folks who enjoy their complex tannins and fruity-oaky tastes, here are some options for placing the gem somewhere that not only give the players the chance to recover the gem and thus revive the wine business (perhaps running it themselves as a fun downtime activity), but also provides a few more ties between different locations in Barovia."
My thoughts were not only that finding the gem and restoring the winery could be a big deal, but there's already a few spots where that gem fits:
1. Death House: since the other gems tend to animate stuff (Wintersplinter, Baba Lysaga's hut), why not have that be the driving force that gives "life" to Death House? Maybe it's a slightly easier win-condition for that Shambling Mound fight in the basement (which is, after all, a Challenge 5 against Level 2 characters, which is just vicious).
2. Tsolenka Pass: this area is boring for the most part. Why not have the gem animate rubble on the bridge or near the forts (or both) and have that work as creatures that attack the party (I think the equivalent of Clay Golems would be balanced for the area)?
3. Have the Heart of Sorrow be the third gem, but with some rituals Strahd cast upon it. If the party can deactivate it, they can thus weaken Strahd a bit, too, for the final fight, which is nicely thematic.
4. Just place it as an additional (or replacement) artifact to the ones that are drawn for in the Fortunes of Ravenloft. Based on where it shows up, add some backstory, and make it a side quest given by the Keepers of the Feather.