I actually was thinking about that and while I don't think they would go for all 6 factions for any given journey (that would sort of go against one of the reasons for decoupling leaders and skins due to not wanting to make 6 new skins every time a new set of leader powers dropped and this would be 6 new leader skins every 3 months) I could see themed sets of factional leaders.
Like an obvious one would be Iorveth vs Roach, you get both of them and some cosmetics for each. Maybe 3 per journey with something like Yaevinn, Jacques de Aldersberg and Dagon as a Witcher 1 themed adventure. Vilgefortz for NG, the Rats leader for SY and Isengrim as major book leaders for another.
Anyway ya I don't think that would be the obvious first pick since I imagine they want to keep these special leaders to a single one per journey right now and that means keeping them neutral, but after you go through Geralt, Ciri, Dandelion, Yen, and Triss* you very quickly run into the issue of not having a ton of great options for characters who can serve as a paper doll leader allot of fans would be excited for. At which point doing 2-3 faction leaders per journey would make a degree of sense even if going up to 6 per would likely not be in the cards.
*Zoltan, while not ST per say, feels like a weird one to have a neutral leader given how much they have leaned into him being a ST character by this point. Same with Regis, plus he would be the 3ed vampire and while Cahir SHOULD be an option as one of the major book hero's, he's not even got a single neutral card to his name so making him anything other then NG would be unlikely.