There's one difference I would bet my soul on that is non-negotiable if Dark Pit is playable.
He
will use the Dark Pit Staff in his moveset somewhere.
Actually, there's two.
His Final Smash
won't be the Three Sacred Treasures. He has no access to them; only Pit does (ignoring that Hades borked them and all...).
If Dark Pit had his own slot in Smash 4, it would be cool if the Dark Pit Staff was his default weapon. If he is an alternate skin, I could see him wielding the Silver Bow.
Since Pit's moveset obviously focuses on close-range combat now, I think Dark Pit's should focus on long-range combat. As for a Final Smash, Dark Pit could fly at the front of the screen, then for a limited time take potshots at opponents, similar to Snake and ZSS's final smashes.
I threw together this basic moveset for the heck of it.
[collapse="Dark Pit moveset"]
Jab (Rapid Fire): Unlike most characters, whose jab consists of a close-range sword or hand-to-hand combo, Dark Pit will rapidly fire a barrage of shots from his staff that damage the opponent whilst pushing them away. (FYI: This move was actually inspired by Saki Amamiya's assist trophy, not Mega Man)
Dash Attack: Dark Pit leaps forward whilst shooting a quick barrage of shots from his staff.
Forward Tilt: Same as his Jab, except he walks forward whilst firing. Very good for approaching, as it allows you to close in on the opponent whilst they are stunned from your shots.
Neutral Aerial: Same as his Jab, except he does it whilst in midair.
Forward Smash: Just like in his game, Dark Pit performs a strong 3-hit combo with his staff. Just like Link's forward smash, Dark Pit's forward smash is all about timing. The first two hits do not knock away the opponent, but do a good amount of damage. The final blow knocks the opponent well out of arm's reach, allowing Dark Pit to snipe them from afar.
Standard Special (Auto Reticle): A reticle appears in front of Dark Pit, which can be aimed with the control stick. Once the player targets someone, all he/she has to do is let go of the Special Button and Dark Pit will fire a charged shot from his staff. The longer the distance, the more powerful this move is.
Side Special (Super Speed): With his free arm held in front of him defensively, Dark Pit charges forward in a sprint, knocking away any opponent whom he collides with. Can be used as a recovery, and is also handy for getting away from opponents.
Up Special (Pandora's Flight): Similar to Pit's Power of Flight.
Down Special (Energy Charge): Dark Pit builds up energy, increasing the power of his attacks. Whilst this move is in effect, all of Dark Pit's moves will deal more damage and knockback than they normally would. If Dark Pit gets hit or takes damage, he will lose the powerup. Otherwise, the powerup lasts indefinitely.[/collapse]