I can't imagine how Ronova could lose to the Tsaritsa, bro

.
I think she'll just be a weekly boss (though even that isn't certain yet), in the same way Arlecchino became a weekly boss.
To be honest, the weekly boss or the main villain has almost never been shown in their region's teaser or trailer. For example, Rerir and Dottore in Nod-Krai, Gosoythoth in Natlan, and the Narwhal in Fontaine were all absent from their respective regional PV.
Instead, the trailers usually feature characters who play a major role in resolving the region's main conflict, such as Arlecchino, Capitano, and Columbina—even though they were once rumored to be the main villains of their respective regions.
Because of that, it's possible that Ronova isn't the main villain. There could be another entity that simply hasn't been revealed yet. And even if the Tsaritsa eventually meets Ronova, there's no guarantee that their encounter will lead to a battle. It could just as easily be a negotiation, cooperation, or something else entirely that we don't know about yet.
As for Piero, I think there's actually a pretty good chance he'll be the main villain. Interestingly, he wasn't shown in the Snezhnaya trailer either.