If I were to make a moveset, this is what I'd do;
First, I'd give him ridiculous air speed and very fast attacks with very little KB. Basically, I'd make Pichu have the best combo ability in the game. They're just really weak combos that sometimes cause recoil,
Then, I'd completely change his specials to entirely egg moves. Why? Because Pichu is a baby pokemon! It matches thematically and helps make him stand out.
Neutral Special: Charge. This move functions much like Aura Sphere. As you charge it, any opponent who gets to close will take continuous damage. However, as you reach full charge, Pichu starts taking damage to. The sphere of electricity surrounding Pichu as it charges is a little bit larger then a Max Aura Lucario's aura sphere to. When you release it, it doesn't launch a projectile. Instead, anyone too close to Pichu takes 35% damage at max charge and Pichu takes up to 2% damage. It serves as one of Pichu's few kill moves.
Side Special: Volt Tackle. A much faster version of Charizard's Flare Blitz, but without the super armor. It does 10% recoil if it lands, but only 5% if it doesn't, and does 20% damage. It serves as one of Pichu's few kill moves.
Up Special: Flail. Pichu throws a small temper-tantrum. In the air, this move functions as recovery. The more damage Pichu takes, the farther it causes Pichu to travel. However, this move's recoil increases with damage as well. At 0%, Pichu takes 1% recoil. At 25%, Pichu takes 2%; and 50%, Pichu takes 3%, and so on. It also does damage that scales as well, and can serve as a kill move when Pichu is at around 100%. Of course, being a recoil move, this attack also has electrical properties, despite being a normal-type move in its own game.
Down Special: Present. A gift is chucked a small distance forward, will lay on the ground for a second, and then explode. The size of the explosion varies, and sometimes, it'll just heal Pichu and the opponent 1% damage. Only one present can be out at a time. Pichu can't be harmed by the blast. The idea is to use this as a spacing tool and an all around annoying move.