I honestly wouldn't mind seeing Dark Pit as a Lucina-style clone. In the same vein as Lucina, I'd see him as more of a fun bonus to the roster than a waste of development time / roster space. After all, if he were as cloney as Lucina, then he'd take barely any time to make I'm sure, and the dev team probably wouldn't include him in the first place until the higher-priority veterans and newcomers were taken care of. Basically - if Sakurai and his team have time to include Dark Pit as a clone, then they more than likely have time to include all of the other, more important characters they intended to have playable. And it'd be kind of nice to have a slight variation on Pit's moveset, I guess, so long as it doesn't cost much development time.
If he had his own unique moveset, then I'd actually be a little peeved. That would imply Sakurai decided to take the time and resources to create a full-fledged character out of Dark Pit, over many other possible choices. In that case, Dark Pit would actually be eating up a significant amount of development time. I think most fans would rather that time be spent on a more deserving character.
As for whether or not Dark Pit is likely, I doubt it. But the ending to Palutena's trailer was definitely suspicious. Yeah, I know: if he was gonna be playable they'd have shown him as a playable character. Perhaps, though, they wanted to introduce this whole "alt/clone" concept with Lucina and Robin's trailer, rather than in the middle of E3's barrage of new content. I'm betting on Dark Pit being an assist trophy or something instead, but I wouldn't be too surprised to see him make it in as a clone.