The stadium in Wiener was quite a surprise, because even if it wasn't big it sold out pretty quickly (or at least, the only remaining seats for months were those with limited view). I doubt they could do Ernst Happel, but the stadium in Klagenfurt might be an option, hence my comment about the stadium in Udine (it's less than a 2h drive from one city to another).
I might know something about Poland but I can't really say anything.
I'm not sure they will do stadiums in Sweden, or at least not Ullevi. Sweden Rock is becoming bigger and bigger and will probably make an offer to the band which would be stupid to refuse. They might play Tele2 Arena in Stockholm too, same as 2018.
Sales for Finland have dropped significantly since 2008, when they sold out two stadiums in no time. I think the lowest point was 2022, when they played to 20/25k people on a big festival on a saturday after 4 years of absence. That being said, in 2023 the shows in Tampere sold out in literally minutes, so who knows.
For Czech Republic I guess it will either be Letnany Airport or Sinobo Stadium again. The 2022 Prague show sold out in 2019, I think, and the two arena shows in 2023 sold out in a couple of weeks (mostly due to the fact that they didn't put all tickets on sale from the start but they were opening sections little by little).
Hungary wasn't that great in 2022, I wouldn't be surprised if they skip it or they return to Volt festival (which, for Maiden, has always sold out faster than other artists).