If the Icies came back (I hate having to say "if" in face of the leak), then they need to be, above all else, easier to desync. Like Rosalina, one of their moves should communicate to the player "Okay, they're separated now, and wherever Popo moves Nana moves but 3 feet away."
I think Belay should be improved to better have this aspect since it already desyncs them already anyway. Whenever you use it, it now sorta acts like Yoshi's Egg Toss, where Popo throws Nana in a similar trajectory. Keep in mind that Yoshi can now throw eggs straight ahead of him, too, so Popo should be able to do this with Nana.
After Nana is thrown to a certain spot, it'll look like they're connected with the rope they use when recovering. While they're connected, Nana will act like Popo does at the distance away from where Popo threw her. From there, you should be able to do Icies shenanigans from the previous games much easier, but with a catch--after being hit, Nana's rope will be cut and she'll stop mimicking Popo to just start running towards him, like past Smash games. As such, it gains its own strengths and weaknesses--it's easier to desync them but it also makes Nana even more vulnerable than before.
Finally, if you decide that you want Nana closer again, just press up-B again and Popo will reel Nana back in with their rope. This also acts as a sort of tether recovery where, if you're trying to recover back onto the ledge, you just throw Nana there, and pressing up-B again will instead have Nana reel Popo back in.
Since it's theoretically much, much easier to desync them, I think Wobbling and their infamous chaingrabbing should be cut as compensation. I'm not sure how to do this, maybe editing Nana's AI where throwing the opponent to her and having her grab him / her will have Nana immediately throw the opponent up or down or something, so she can't pummel. But I definitely think that chaingrabbing wasn't an intended design choice for them and the concept of chaingrabbing is polarizing (pun not intended) as it is, and desyncing should be their central strength instead.
Keep in mind, however, that while this is not coming from an Icies main, exactly (use this knowledge to either disregard what I have to say, or not), it's still coming from someone who
A) wants every character's core concept understood and able to be utilized within about 5 minutes (Icies in past Smash games have been notoriously hard to use, for little gain besides things like auto-win techniques, i.e. Wobbling), and
B) doesn't want any character to be overlooked by most simply because they're much more difficult to use than another character using the same concept (Rosalina is inevitably gonna be compared to the Icies and seen as easier to use as a result).
So, having watched this thread for a while now, I'd like to hear some thoughts on this change. Is it better? Is it worse? A friend of mine doesn't like the change simply because it, well, changes them, but I disagree because it makes their overall design philosophy cleaner and easier to get a grasp of, while also opening up new possibilities on top of old Ice Climber techniques. I ask again here because I'm sure a higher repository of Icies mains reside here, and I'd like to ask beyond just my friend to get a better sample of opinions.