Here we go. I have conjured up some major but unique changes that both stem from Melee, but make Pichu incredibly different from Pikachu.
Here are changes based off of Melee
Upsmash is weaker than Pikachu's
Fsmash is stronger, and is a multihit move
Dsmash isn't electric and is a 1/2 hit move
Uair tail spike is gone
Utilt has less knockback
Dtilt has less knockback
Less range overall
Shorter Short hop
Up B gains slightly more distance but less controllable
1 or 2 frame lag on QA
Less killing potential on nair
Here are major changes that makes Pichu unique from Pikachu:
You can no longer kill with thunder from the top. Thunder is now a weak spike that is consistently recoverable until at a very high percent; in other words, it sends you down, but isn't strong enough to gimp until a high percent. The thunder will set up into a combo (upsmash? uair?) if the opponent is hit lower on the bolt.
The base hit thunder (a thunder that hits Pikachu/Pichu), will also not kill until higher percents. I see this having an upward hitbox. I am not sure whether it should be combo able into a bolt hit thunder. Of course, proper hitstun and knock back would have to be tested. This can kill at extremely high percents (170?)
Shorter double jump for a SHFFL-esque QAC. I see this having much combo potential, but in return has a worsened recovery with the shorter DJ. To balance things out even more, 2 or 3 percent self damage on each QA.
Some one mentioned this on the IRC. I'm not sure, but a 3 part QA that has shorter distance on part.
Dtilt that hits up.
A ground hit Dair that sends them up and/or combos into an fsmash.
Fsmash setups are needed, as I think it needs to be similar to Melee, where Fsmash was incredibly good, and probably one of the only consistent ways of killing at a lower percentage.
Basically, I see Pichu as an extremely good damage racker, but when it comes to killing, you have to rely on Fsmash. Killing options from thunder are gone, but thunders now can start combos, combined with better QACs (with a trade off). Of course, usmash will also be able to kill, but to a lesser extent. I also think combos should not stem from uair, or utilt to differentiate them even more.
Also, I have contacted BigSharkz, and making a Pichu is his first priority now. I have him on MSN, and it looks like he says it is very possible.