Diablo Immortal has teased on their Twitter account about a possible crossover with Hearthstone with a short caption and a picture advertising the crossover.
Hearthstone or Diablo?
After the StarCraft crossover mini-set, Heroes of StarCraft, it seems likely that Team 5 would want to branch out their horizons to try new crossover sets with other Blizzard IPs, but the picture being painted is not as clear as it seems.
We at Out of Games are speculating that this is teasing Hearthstone-based content added to Diablo Immortal and not the other way around. Our first hint towards this is the fact that this is being teased by Diablo and not Hearthstone, and it would make more sense to advertise the crossover for the game that the event would actually take place in. It wouldn't have made much sense for StarCraft to be the first to advertise the Heroes of StarCraft mini-set because it wouldn't be relevant for people who play StarCraft but not Hearthstone. Likewise, if this was another crossover set, it wouldn't be relevant to people who play Diablo but not Hearthstone.
Additionally, if Diablo did want to advertise this hypothetical crossover set, it would make more sense to advertise it on the Diablo Twitter account instead of the Diablo Immortal one. Having the announcement hosted by the account for the spinoff game implies relevancy specifically for the spinoff game. A crossover with just Diablo Immortal as opposed to Diablo as a whole wouldn't make sense unless various entities only from Diablo Immortal were present.
We believe another hint lies in the ordering of the games mentioned in the crossover. In this announcement, Diablo Immortal appears first then Hearthstone. All of the promotional material for Heroes of StarCraft, the trailer for example, listed Hearthstone first. This may appear to be entirely circumstantial and the games were simply listed in alphabetical order, and that might be true, but it makes the most sense to list the most relevant game first. The Heroes of StarCraft mini-set is obviously most relevant to Hearthstone as its host game, so it gets listed first. In a similar vein, listing Diablo Immortal first implies it is the most relevant game, and therefore likely to be its host game.
Then again, this could be entirely off-base and it truly is a Hearthstone event after all. If you have differing opinions than us, feel free to share them in the comments down below.

Comments
"but the picture being painted is not as clear as it seems" since it clearly wasn't painted but AI generated...
Doesn't look AI to me. It looks like a rough painting that didn't have any fine details added. Maybe Tyrande's face is, but I don't see any obvious AI artifacting.
Clark Hellscream made an entire Video about this. The Hearthstone Cards have the Mana on the wrong side, while the back swirls are switched in Tyrande's hand, there is no plate for the meat and a few other small details that wouldn't have happened if it was a real drawing.
To me they seem like mistakes a human WOULD do tbh, and I think the background is too.. sensible to be AI. BUT ultimately it's impossible to say for sure.
This COULD be an AI image that was then worked on by a real artist, which is honestly the most likely (and depressing) option.
Everyone seems to think they are suddenly an AI image expert these days without understanding anything about it, but that's nothing new.
That's a damn shame.