Regarding the topic of Dark Pit:
He could be a regular alt. He could be a stage hazard. He could be a stage boss. Or, he could be playable.
Let's focus on the idea of if he were playable. How could it work?
Well, first things first, he has to be visually distinct from Pit, alt colors included. I think this can be achieved, first and foremost, through his WINGS -- Pit, and all of his alts, will always have white wings, while Dark Pit, and all of his alts, will always have black wings.
Second, take the Sonic the Hedgehog approach -- much like how Sonic and his alt colors are varying shades of blue, Dark Pit would always be "dark" by making his alt colors lighter and darker shades of black. Pit, meanwhile, could come in a whole rainbow of colors.
Beyond that, tweak the animations to reflect cockiness in Dark Pit, and add extra effects like red sparkles to Dark Pit instead of the blue sparkles we see on Pit.
Now, as for what a playable Dark Pit could offer: He'd essentially be a low-effort, Lucina-style clone that would give Sakurai the opportunity to incorporate more of the many, many wonderful weapons he created for Kid Icarus Uprising. After all, in that game, Pit encounters Dark Pit on a number of occasions, and each time Dark Pit is wielding a different weapon. Swap out Pit's Guardian Orbitars for another set of orbitars, with different properties; swap out his Upperdash Arm for another arm that handles differently; and so on.
Essentially, playable Dark Pit would be a way to represent more of the weapons from the world of Kid Icarus Uprising. There were more than 100 of them, IIRC, including some truly clever designs and effects. I'd love to hear the sickening roar of the Ogre Club, for example, a fleshy spiky club that Dark Pit uses against you in Chapter 6 of Uprising.
