Pichu was included in Melee in order to represent the Gold/Silver generation of the Pokemon franchise. That's why it was cut from Brawl, and there were no other Johto Pokemon (even in Pokeballs, excluding legendaries - Togepi is the exception) in noteworthy appearance in the game - G/S was no longer apparent during Brawl's release. Kanto got representation because it is the original, and because of the Firered/Leafgreen remakes, Ruby/Sapphire is in because it was the one generation skipped over between the Smash Bros. releases, and Diamond/Pearl is in because, obviously, it is the current generation of Pokemon games. Johto is the least represented of the four Pokemon regions in Brawl in every way - Pokemon, references, and music.
But now, Heartgold and SoulSilver, the remakes of Gold and Silver, are coming out soon. And since Firered and Leafgreen were represented by the Pokemon Trainer (by the character design - he even has the FR/LG-debuted Vs. Seeker), it can be concluded that Heartgold and Soulsilver will be represented in the 4th Smash.
Pichu still remains as the poster Pokemon of the 2nd generation of games - many updates of HGSS on Japan's Pokemon Sunday had a person in a Pichu costume in order to endorse the games. Also, in an event for HGSS, there is a specific Pichu sprite, appropriately called the Notched-Ear Pichu, that can be acquired. This Pichu cannot be evolved or traded, and is the only Pokemon confirmed so far to have a specific sprite that differentiates from all other Pokemon of the same species, gender specifics included. That alone solidifies that Pichu is the one thing you should remember if you think of Gold and Silver.
So, when SSB4 comes around, I can expect the HGSS games to be represented. And considering the massive attention it has had in the HGSS updates, and the fact that it had an important role in the recent Pokemon movie, just like Lucario did when he showed up (and there is evidence to believe that the Smash Lucario is the same as the movie Lucario), leads me to believe that Pichu could come back as a Smash character in order to promote HGSS, with the Notched-Ear Pichu as either a different costume, or the Pichu character altogether.
tl;dr - Looking at evidence of representation in the Pokemon franchise, and making predictions of future representation, if the Johto Pokemon Trainer doesn't get any rep in SSB4 (which may be unlikely anyway, since the Pokemon precede the trainer in rep in every region, which is why Pikachu got the roster spot for 64 and Melee), being the poster Pokemon for the GS games, Pichu has a chance of coming back for the Smash games.
Bash away.