Even though there's no way in hell it's happening, I feel like Sakurai and his team could competently bring Pichu back as a veteran / newcomer by re-introducing not one, but two Pichu using a tag-team mechanic like Rosalina & Luma or Ice Climbers. That is, the Pichu Bros.
This isn't without precedent in universe, either - GAME FREAK seems to have caught on to the fact that a standalone Pichu is usually inferior to Pikachu, so they'll usually team the notable ones up in duos to compensate. There are two major Pichu duos in-canon: the Pichu Brothers (or Pichu Bros.), which first appeared in Pikachu & Pichu, the short attached to the third film.
Much more recently, there was an unnamed team-up between two Pichu: the Spiky-Eared Pichu ("Notched-Ear Pichu" in Japan), and the Pikachu-colored Pichu, which is a shiny Pichu. It's an event taking place in HGSS's Ilex Forest where if you bring the Pikachu-colored Pichu to Celebi's shrine (the shiny Pichu obtained through wi-fi distribution), the Spiky-Eared Pichu would join your party.
Pichu Bros. (the SSB4 variant) would be a composite of these two team-ups: one Pichu would be a shiny and be slightly stockier, nicknamed "Big" (after the movie 3 one). The other would basically be the Spiky-eared Pichu in design and be a little more dainty-looking, nicknamed "Notch" after the Japanese name. Big is a male and Notch is a female, which stays true to the genders HGSS distributes them in. For the sake of an easy name, it's still Bros. despite the male-female thing. "Pichu Couple" doesn't really have the same ring to it.
The dynamic here in contrast to the other duos is that there's no seniority or "dominant" Pichu: instead, Big dishes out higher knockback but less percentage infliction while Notch uses electrified versions of Big's (non-Special) attacks, which - while not in possession of as much knockback - deal out higher percentage, at the cost of slightly damaging Pichu Bros., albeit not anywhere near as badly as Melee handled it. This creates a dynamic where Notch starts dishing out damage, and Big can finish the job with higher-knockback attacks. Specials remain the same regardless of who "leads," as they utilize both characters. Additionally, in contrast to the other two duos, if one Pichu gets knocked out, the other can continue fighting indefinitely, unlike the Ice Climbers. That being said, the other Pichu will not respawn until both Bros. are KO'd.
Similar to Mega Man, rather than restrict Pichu to four real "moves" via Specials as is the case with Pikachu, Pichu Bros. utilizes a variety of attacks based on the games - Pichu has received a surprisingly high number of special / signature moves throughout Pokemon's history. As for its moveset:
Neutral B
Both Pichu use Reversal, a counter of sorts. Big stays floored, swinging Notch around as the two build up speed and momentum. This deals basic physical damage, with the amount of hits increasing the longer the move is held down. If attacked during Reversal, Notch latches on to the assailant and electrocutes them before hopping off and going back to Big's side. If Reversal completes its "charge" animation, Big will throw Notch off into the distance, which can be extremely effective as a recovery move. The throw has a consistent arc that can be changed depending on controller input, similar to Yoshi's egg tosses.
Side B
Big uses Wild Charge, a weaker variant of Volt Tackle in the games. In Smash, Wild Charge would work like a hybrid of Pikachu's Skull Bash and Sonic's Side B, functioning as a quick horizontal movement option with on-the-dime turning. It can also be charged. During the attack, Notch will ride on Big's back, but if charged, Notch will use Swagger (a move that sharply raises the target's Attack, but confuses them) on Big, increasing the damage output and move's range at the cost of not being able to turn the attack around. Swagger, for the sake of convenient animation, would just be Notch striking silly body-building poses while Big gets covered in RSE-styled buff filters.
Up B
The upward special uses a variation of Double Team taking a few liberties with the game and anime usages. Pichu Bros. send out a veritable fountain of doppelgangers to propel themselves upwards in a very floaty fashion similar to the air currents in the Subspace Emissary; the Double Team clones cannot inflict damage and fade into nothingness fairly quickly after their use, but they can push players away slightly like Mario's FLUDD. While it renders the Bros. helpless afterward, it's entirely possible to screw with enemy aerial game by pushing them well beyond their recovery ranges.
It's probably worth noting that there's a slight kink to their recovery method with regards to edge game: because of how small they are, both Pichu Bros. can hold on to a single ledge concurrently, though two Pichu from enemy Bros. players can't share a ledge. If it's a reasonable distance away and one Pichu manages to reach the ledge, the unsuccessful one can use a simplistic "Thunder Tether" (let's just say it's Thunder Wave) to gently float to their teammate, though the Bros. don't actually have a tether recovery available. If both successfully reach a ledge, they also have cute little animations getting up off the ledge, where Big would get up first and pull the physically weaker Notch onto the stage, for example.
Down B
Pichu Bros. use Volt Switch! While it's not in the traditional trainer-controlled sense, the down special switches who "leads," with the differences between the aforementioned attack differences Big and Notch have. It deals minor damage to the Bros., being an electric attack, as well as for balancing reasons - Volt Switching is meant to be a carefully-guided action, not something for constant use. In the air, it can also be used to "round up" the Bros. similar to Olimar's whistle.
Grabs
I won't go into detail on every single pummel and tilt, but the tag-team nature of the character justifies the grabs a little more easily. Notch stuns the enemy character with Thunder Wave, while Big hops on to the head / torso of the opponent and uses Doubleslap.
The Bros. have some very minor magnetokinetic abilities such as the aforementioned "Thunder Tether" for the sake of spicing up gameplay compared to the very vanilla style of Pikachu. If it's any consolation, Spiky-eared Pichu
does come equipped with a Zap Plate in-game, which juices up the power of electric attacks. This could even show up in taunts, where the Bros. pull out their respective items: the Pikachu-colored Pichu holds an Everstone in-game, which could double as a backhanded taunt to Pikachu for letting himself evolve.
You're probably wondering (if the wall of text didn't already make you forget), though: what does Down Special do if you can fight as a single Pichu indefinitely? Well, both Pichu just so happen to have some very Smash-friendly signature moves in their HGSS versions. Notch has access to Pain Split, which in-game takes the sum total of the user and target's HP values, and distributes them evenly among both players. It'd work like a mix between
that one move in Super Metroid's finale and Diddy's facehugger attack, where Notch latches on to the enemy assailant and rapidly evens out their percentages until knocked off. This also compliments Notch's playstyle on an individual level, allowing for her self-harming attacks to actually become an asset against enemy players. On the other hand, Big has access to Endeavor, a move that reduces the enemy's HP to that of the user's. In Smash, Endeavor would animate a bit like Mewtwo's disable with a longer windup, where Big took a wide stance before sending a white pulse of electricity out in front of him. If it hits a target, it would automatically up the victim's percentage to that of Big's, and also briefly stun them, allowing for it to become a lethal setup for a finisher.
As for the Newcomer video, as much as I'd love to spend countless hours somehow making an animated equivalent, I enjoy having a life. I have to imagine it'd somehow canonize Pichu from Melee becoming Pikachu in Brawl hence the wall-jumping addition, goggle alternate costume, and Volt Tackle as a Final Smash. Eventually, this would have some kind of "rags to riches success story" effect on Pikachu which would show him doing dumb Pokemon stuff, inspiring some onlooking Pichu. The Pichu Bros. would have a training montage involving ridiculous stuff like Pokemon-Amie or fighting in Pokemon Gyms before eventually getting up-to-par with the rest of the Smash fighters. Cue montage of ridiculous tag-team Pichu shit.
I have no idea what the Final Smash would be, but I have to imagine a Dedede-styled "Big Town Dance" where they summon the
Pokemon Clubhouse from the anime short in the background and debris like balloons and runaway tires as well as Houndour, Azumarill, etc. come out of the woodwork to wreck the shit of any enemy fighters.
That being said, if they got their own stage, I feel like the Pokemon Clubhouse could probably be a pretty cool unlockable battlefield.
Couple of interesting tidbits regarding potential inclusion (and their relevance):
- We have two Pichu duos already existing in-canon.
- The voice actor for "Pichu Little" (the smaller of the two anime Pichu Brothers) also voiced Pichu in Melee.
- Pichu as a species has received a ton of signature moves being that they could inevitably be "passed on" to Pikachu, the mascot, while also helping alleviate the whole self-harming angle the Melee utilized.
- Having a male / female pair would not only up the female count (which Sakurai's shooting for), but also add in a new female non-humanoid fighter, and finally bring back representation for Gen II of the series, which is probably going to have some demand in SSB4 thanks to HGSS.
- The Spiky-Eared Pichu actually has its own unique encounter theme,
which I took the liberty of mixing and arranging to better fit the currently existing SSB4 music. In contrast to Pikachu "playing it safe" with pretty straightforward anime-styled music, the Pichu Bros. would use upbeat, bubbly synths in addition to some mild background guitars and chiptunes in reference to Generations 2 through 4's musical styles.