Well, first of all, they are only the second two-in-one character. And, while I acknowledge that's not exactly "new," I definently think its a point in Pichu Bros. favor. When you consider that the final Project M Roster can have up to 46 characters, having only one of a character archetype (in this case duos) is pathetically low. Adding just one character of that archetype can alleviate this problem, if only by an extremely small bit. This character can also appeal to a wide range of people. Pichu fans can be happy because he comes back. I acknowledge they are a very small minority, but also consider that making Pichu competitively viable via the Pichu Bros. (this is because they are getting twice the power due there being two of them, but at only regular recoil damage for each Pichu Bro.) could give them the potential to be rather popular among the competitive community due to his archetype having an extremely high learning curve because of the fact you have to ensure the safety of two characters and the fact they are a rarity in Smash Bros. because he is only one of two of his type, as well as the fact that his doubled power (which would likely be a major part of why he is competitively viable) relies on the fact that both Pichu Bro.'s attacks must connect in order for the double power to take effect, which requires substantial reading skill, a high amount of accuracy, and extremely good timing. In addition, he is different enough from both Pikachu and the Ice Climbers to potentially gain more popularity. Obviously, one of the main differences between the Pichu Bros. and the Ice Climbers is the fact that the Pichu Bros.'s moveset would be almost totally cloned from Pikachu. When seperated, both the Pichu Bros. and the Ice Climbers experience a drop in overall power. However, differences are furthered by the fact that a lone Pichu Bro. could still be somewhat viable, unlike a lone Ice Climber, who, if knocked off stage, practically means death due to his pathetic recovery. This is because Pichu has access to Agility, allowing decent solo recovery. But, to compensate somewhat, and also furthering the Pichu Bros. even more from the Ice Climbers, recovery as a team as the Pichu Bros. would be rather difficult, considering that only one person can grab a ledge, odds are one Pichu Bro. would die as a result of having Agilty as a recovery move. Ice Climbers, however, can both survive due to the way Belay works, if I remember correctly. Once again, this just distances the two pairs even more. Also, when desynched, the Pichu Bros. can still recover when seperated during a desynch due to Agility, whereas with the Ice Climbers it is practically impossible to recover when separated and desyched. This same principle of being able to recover when separated also applies to basic separation, especially when it is forces separation. This makes the Pichu Bros. able recover if knocked off-stage when forcibly separated, unlike the Ice Climbers. When it really comes down to it, the only real similarity between the Pichu Bros. and the Ice Climbers is that both are two-in-one-package characters. The Pichu Bros. would provide a completely different learning experience and play style from the Ice Climbers. In my opinion, these arguments should justify a place for him, but I'll go on in case you doubt it. They are also somewhat different from Pikachu. While the Pichu Bros.' moveset is almost completely cloned from Pikachu's, keep in mind there are two of Pichu, a drastic difference in and of itself. This makes learning to play the Pichu Bros. a completely different experience from learning to play Pikachu. You have to develop new skills and have excellent focus in order to use a two-in-one-character, especially when it comes to desynchs. Desynchs are important to the Ice Climbers, something that would probably carry over into the Pichu Bros. The different skills it takes to use a character that can desynch differentiates using Pikachu and Pichu drastically, and when you consider all the other differences between the two Pokemon, as well as the accuracy and timing it takes for the Pichu Bros.'s double power (something necessary to make it competitively viable) to be used properly, it becomes a completely different learning experience and play style, just like as with the Ice Climbers. Finally, let us not forget Pichu's most unique feature: his self-damage system. Pichu's gimmick is the fact that it hurts itself whenever it attacks, generally speaking. No other character in the entirety of Smash has this gimmick. Putting the Pichu Bros. in Project M brings back this truly unique gimmick, and allows them to create a character that depends on accuracy and timing unlike any other character. This is something very new that they bring to the table, because literally no other character in the game has the gimmick that the Pichu Bros. would have. Technically speaking, since the Pichu Bros. could have the Pichu from Melee as Pichu Little, they could be considered the only characters in Smash with this gimmick. This alone is something incredibly new that they bring to the table. In addition, they can be somewhat unique in that, while being small and fast, they also are rather powerful when it comes to blow-for-blow power. Maybe not necessarily overall power, but certainly blow-for-blow power. I say "somewhat unique" because of Squirtle, who is similar in this respect. Even then, they would only be the second character Project M like this, and to present a small difference between Pichu Bros. and Squirtle in this regard, the Pichu Bros.' wallop only truly kicks in when both a Pichu Bros. are present and both land the hit. So, let's review the new stuff that they bring.
-They're only the second of an uncommon archetype in Smash
-They are completely different from the Ice Climbers, with the only thing they truly have in common is the fact that they are both two-in-one characters
-They provide a completely different learning experience and play style from primarily Pikachu, but also the Ice Climbers
-They would be the only character in Project M to have their unique self-damage gimmick
-They depend on accuracy, timing, and reading skills like no other character, because it is this power that makes it competitively viable
-Unlike the Ice Climbers, they can recover when forcibly separated or when only one Pichu Bro. is left
-They are somewhat unique in the fact that, while being small and fast, they also pack a wallop when it comes to blow-for-blow power
-The difference between their blow-for-blow power and Squirtles's is the fact that the Pichu Bros.' power only truly kicks in when both are present and both land the hit
Overall, I think the Pichu Bros. could bring a lot of new stuff to the table and provide a unique learning experience and play style different not only from Pikachu and the Ice Climbers, but from the entire cast as a whole, because of the unique skills, gimmicks, and differences that they hinge on.