Not a fan of the perma-party idea if it means the non chosen NPC's become inaccessible. I think the best solution would be to go the BG1 and 2 route, namely that some companions needs to be recruited when you meet them or you need to complete their specific requirements or quests otherwise you risk permanently losing them, others can simply be ordered to leave the current party and then simply 'go live their lives' meaning you can find them at an appropriate location (e.g. Astarion in his house in BG, SHadowheart in a Shar/Selune temple or crypt, Lazael somewhere outside of the BG city walls near a gith encampment in the woods, etc etc).
I think they could pull it of in theory, but I think they wont succeed in writing a convincing narrative that makes it feel and flow naturally instead of some narrative deus-ex machina trick which feels gamey in the sense of 'at the end of the tutorial you need to chose your definite party but instead of a menu they made a cinematic out of it'. However, I fear that we're too far into writing and development to change course as it seems one of those stupid things Larian somehow keeps holding on to without good reasons. An in between would be to at least not completely remove the non-chosen NPC's from the game/story-line, like at least give them a cameo at a later point in the game (as antagonists or even as friendly 'non party' NPC's like wyll when you first meet him at the grove fight) and don't let them die permanently...
