The trailers of Rosalina and Luma seem to suggest that Luma is a significantly less complicated "partner" then Nana has been previously, at least that is how it looks in my opinion. So I think a significant change is inevitable for the ice climbers.
I think the emphasis on new "alt" costumes in the game might be hinting that Popo and Nana are going to be separated and instead be alt costumes of each other, the character would probably be called "Ice Climber" to keep the naming convention. This might be necessary due to hardware limitations, plus since so many characters are getting altered move sets from previous games it isn't hard to imagine the Ice Climbers receiving new moves to make up for being separated.
We now have confirmation that both WFT and Villager have gender swapping "alts", it will be disappointing if the Ice Climbers are separated but it will be a far easier pill to swallow if they keep revealing more fan favorite "alt" costumes. The logic behind these alts seems to suggest that the character's "alt" has to maintain the same name as the original character, since Ice Climbers have never been called by there name in character selection the "alt" idea would meet this criteria (assuming the name thing is a criteria at all). I wouldn't be surprised if the concept of alt costumes was created entirely because the Ice Climbers have to be separated, and they may be revealing them slowly to curb the sting of a changed Ice Climber character.
Oh and First post! Yay!
I could see it going like that - simplified Nana, or seperating the Climbers into solo costumes. I could adapt to either, really. I hope they keep them as a pair, though. Maybe simplifying Nana wouldn't be so bad. What if Sakurai had Nana desynched by default? Keeps the authentic feel, but make Nana almost like a part of Popo's body.
Still, though, it's hard to picture in my mind how that would work out. I, like most people, would just prefer if they were able to have them come back more or less the same. The only issue seems to be the need for AI for Nana, since she needs to be able to strive to return to Popo if they become seperated.
Somehow linking their bodies with an invisible "tether" could be the trick. If Popo gets launched, so does Nana. But if Nana gets hit, does Popo take damage, too? All very complicated.
So many ways it could go. Hopefully ICs are one of the character reveals at E3!