Ciri Nova is the single worst card in the game. It optimizes the worst aspects of Gwent right now. It makes last say incredibly important, making problems such as the coinflip and spies that much worse. It limits deck diversity - when this card is a viable finisher why bother with other faction finishers such as Bloody Baron or Ciri Dash (I know its neutral but you can build a deck around it, like constantly discarding it or consuming it)? Not to mention, when something works better with it's limitation, it will instantly become a better card - Skellige for example can thin enough that they get the important cards anyway, and the limitation is not that limiting. It's also annoying that the limitation isn't something the enemy can control, there's no agency there - if you have last say you can scorch and that's about it. It's not like you can add cards to your opponent's deck or something.
Personally I don't think lowering it to the 15-20 range is too big a nerf. It probably wouldn't see much play then, but to be honest, good. Seems to me like every other game right now is a Ciri Nova game. The other idea I'd do is to have it boost by 8 at the end of every turn, rather than instantly boost to 25 or 22. It would take 3 turns to get the current value, but it could go higher. It would remove the last play ability of it, give an actual chance for the opponent to do something and to me feel like a card with actual agency to it.