I guess I need to give my two cents on this.
Dark Pit's most likely gonna be merely Pit's alt costume. Remember that there was no black Pit costume during the demo, meaning that it's likely they've been hiding stuff from me. The only unique thing bout said costume would be the voice, although different taunts would be pushing it. If he was to later get the Lucina treatment, he'd probably be stronger, but slower Pit variant which isn't likely to happen yet (trust me). If he does get lucky and gets his own slot (I hope he doesn't), but he really will be the typical slower, but stronger type that uses the Silver Bow.
Besides, the only thing that this has confirmed is that Marth and Lucina have different traits and abilities despite being similar to each other, and therefore, Lucina gets her own slow. Honestly, if there is at least one unique costume per character, I would assume that Lucina is Marth's unique variant with different traits.