Hmm. Maybe. Ithlinne isn't the one who's OP, it's that the game has become 'swarm.deck: The Witcher Card Game'. If the metagame changes she'll just be another strong ST card. It's important to separate card strength from situational factors like the meta when determining if it needs a nerf. If Alzur's Thunder didn't do 9 damage, and tremors didn't create 20-30 point swings due to mass muster/create, she'd just be another 20 pt gold, which seems to be a standard level for golds these days.
If CDPR is happy (ish) with the direction they're going in, then yes she'll need a nerf, because the spells are getting obscenely powerful now.
As to the thread topic... aren't they ALL must-have? The only ones that are really situational and dubious seem to be Iorveth and Aglais, who are both still really good. ST's always had a good selection of very strong golds.