The problem with Ice Climbers being on the 3DS is the fact that both Popo and Nana have their own individual AI's, as 4 IC players would be an overload of how many complex AI's the 3DS can render at once. Pretty confident that's the problem, i see people suggesting make them 2D sprites on the 3DS version. But that's not the issue, the 3DS can render multiple character models on screen at once, it's the AI that's the problem. Smash Run proves this, as there are dozens of enemies on screen at once. Because their AI's are simple, player AI's on the other hand require a lot more hardware resources.
Though it is naïve and very optimistic to believe that they just didn't have time to make Ice Climbers work, it's not unreasonable. As Sakurai's wording when he addressed the situation really makes it sound like that was the case.
"I had the team innovate a lot to try and get them working, but we had to give up eventually."
This can in fact mean two things, either they gave up because there wasn't enough time, or they tried their hardest and it just did not work. And i think the latter would've taken up quite a lot of the development schedule of the game. Which is why i'm leaning towards the fact that they just did not have enough time.
I think Sakurai and the team should find a way to get Ice Climbers working on the 3DS version first, before they make any drastic decisions like making them exlusive DLC for Wii U and New 3DS. While that's not the worst idea, it's not the best one either. It would cause a weird split in the userbase of the game, like, how will the Ice Climbers work Online if they can only be played with the New 3DS? So if exclusive DLC were to become a decision, i put my money on Wii U exclusive DLC.
If Exclusive DLC were to become a thing for them, only a vocal minority would complain about it. But let's be real here, this happens with almost everything. Need i remind you, there were a group of people that protested against For Glory mode when it was first announced. So no doubt Exclusive DLC would cause the same uproar, but it'll die down eventually as people just get over it. Besides, i've actually seen
more people want Ice Climbers as Wii U/New 3DS exclusive than naysayers.
What i really want to happen though, is that Smash for 3DS, the game as a whole, gets an upgraded re-release version made specifically for the New 3DS. With features and content that wasn't possible on the original 3DS, you can clearly see Smash for 3DS pushes the handheld to it's limit. (Extremely long boot up loading time, Home menu during game takes much longer than other games, Web Browser not available during game and etc.) Yet, it is still extremely far away from the Wii U version in what it offers. A new re-release version for New 3DS would be a great value, as it would make the handheld counterpart of the game much closer to the Wii U version. Upgraded re-releases is in Bandai Namco's blood, too. This would be the best way to get the Ice Climbers back. because the roster parity still consists within this decision.
But, i have been thinking a lot about this. What way is there to implement a working Ice Climbers to the 3DS without drastically changing them from their previous incarnations, such as splitting them into a solo Climber? Like i elaborated on earlier, the problem is more than likely the AI's they have. So i've come up with a concept that they could possibly use to fuse Popo and Nana's AI into one. Duck Hunt Duo is proof you can have two fully interactive character models in one AI.
I made these illustrations for my concept:
Tie Popo and Nana together. This way, they can share the same AI and are always in sync. Sure, this would eliminate the gimmick of Nana desyncing from Popo at times. But their core moveset would still be the very same.
^ How this execution would make them look during gameplay, Nana stands behind Popo instead of next to him like in previous games. To make sure you can see her visually, because they are completely in sync and Nana no longer desyncs during quick movements.
Anyway, that's just what i think, and what i came up with. I may be wrong in the end, but it's a risk i'm willing to take.