First off, while I am okay with people disagreeing with GamerGuy09, sometimes I think the way it is being handled is rude and rather harsh. I don't think he is a delicate creature, but I like the Robin thread because we have been mostly kind thus far.
That aside, I'll state that I think I think a pair up is an EXCELLENT character idea. To address some points:
1) "Robin doesn't need to be with Chrom to be interesting"
Of course not. You could theoretically have every lord in the game and they could have wholly unique fighting styles. The thing is, there is still a lot of value in a potential unique fighting style that is far unlike any other. For example, if your special button calls the other character to assist while you still have control over the active character, there is a realm of combo potential that no character has. For example, if a character has a multi-hitting attack, you can call in the supporting character to induce more hitstun to continue the combo. Not even Rosaluma is like this. Of course Robin will work just fine alone, but I think there is even more potential in this fighting style and she could be an even greater character.
2) "Pair up was just a one game thing"
Yeah, it was. Does that matter? Smash Bros has tons of one-offs and things based on just one game. We have two characters, Ness and Lucas, from a series that is DEAD to the developer. Also, even though I think this argument was already insignificant, it's not the only one. The pair up system itself represents the support system. The close bond between your characters has been a flagship feature of Fire Emblem for over a decade now. It represents a more physical aspect of the support system that has been there for ages.
That said, while it is a cool idea, it is not going to happen.
I would say that the developer has alluded to trouble with CPU limitations, and I think this has played a part in the lack of transformations as much as design philosophy itself. In theory they could hope people play no more than two Robins on simple stages, but that is not going to happen. They are going to prepare for the worst of everything... four Robins all calling in Chroms to assist, on a transforming stage, with physics-intensive assets bouncing around... just about everything to fry that poor chipset of the 3DS and cause massive frame drop. There are lots of ways on the graphics side to reduce strain on that front, such as what they are doing with Pokemon and Assist Trophies, but the CPU issues remain.
And no, Luma and Pikmin are unfortunately not at all an equal comparison. Their bodies are simplistic and have very few joints. Pikmin have very "skippy" animation movements. They have very few joints and have very basic collision protocol (IE they don't need to calculate for bounce) for a reason. Luma, on the other hand, is clearly cheating out collision entirely, which was a smart decision. The Luma deliberately has a large terrain pushbox as to not have to deal with a lot of CPU strain on ramps and stuff. The movements of the asset largely relies on very basic distortions and the faces are largely replaced on the spot to allow for expression.
That said, from a technical standpoint, it is still possible from my knowledge, but I won't understand everything going on with the game without having had time to play and study it myself. I would not doubt that the pairing system would be the first thing to come to mind for a character if Sakurai were to play the game, but if it doesn't show up in Smash Bros, it didn't show up for a very good reason.