In general, splitting up a unit doesn't seem like Sakurai's thing.
Pearl and Marina are a pair. The Champions are a four-person unit. The house lords represent three different stories (or four, when you factor in Edelgard's alternate route). They are conceptually grouped together in all of their marketing appearances.
Ah, but what about Rex and Pyra being a driver-blade duo, you say? Sakurai says in the presentation video for Pyra that he did try to make Rex playable alongside her, like the Ice Climbers. And in Famitsu he wrote that he also considered having him in the background like Pokemon Trainer. But neither was technically feasible.
And in the end, Pyra and Mythra are co-equal to Rex as story leads, anyways. They are not different story campaigns, as with the house lords where you only experience one side in a given playthrough. They both go on the same journey.
So I can see how Marina, Urbosa and Claude are a bit complicated in this respect. Twintelle really was the least complicated choice, but she was probably undercut by how complicated she is as an expression of the ARMS concept. Sakurai would have to explain that this rep, for a game called ARMS, doesn't use her actual arms, but uses weapons called ARMS on her hair. It probably struck Sakurai as overly specialized or needlessly convoluted when Yabuki's choice, Min Min, uses her actual arms and even transforms them for variety.
Hopefully Nintendo has some major new Black characters in the works who will receive top billing in BotW2, Odyssey 2, Splatoon 3, etc -- characters who stand on their own and don't bear the baggage of a team or group.
