On the Pittoo, he's indeed relatively cheaper to develop than, say, Palutena. Even supposing Sakurai changes every move of his so it's different than whatever that Pit uses, chances are there are some ideas for Pit's moves that were unused for various reasons. Dark Pit can reuse majority of those more or less verbatim. However the thing is that difference of money is not that big if Sakurai doesn't want to have a straight clone, and he probably doesn't want another one in the collection. Even jumping animation of Luigi was changed despite him being quite different from Mario as of Brawl. Also note that while Sakurai decides over the game's roster, he would probably be consulted by marketing teams, and those probably remember reactions to Sony revealing a Pittoo-like in PSASBR. (Yes, there are some differences between this situation and that, but there are also similarities.)
Supposing that Sakurai doesn't go self-crazy, I expect the SSB4 to not contain "episodic" characters from KI:U. What constitutes an episodic character? Well, let's see in how many levels of KI:U did they appear (in conversations or otherwise). If my calculations aren't wrong, we can see some kind of magic barrier at 3 levels, with Magnus, Pyrrhon, Chaos Kin and Pandora ending up somewhere around there. Heck, Medusa got in to total of 4 chapters, but she has the privilege of being primary villain of the first game.
So far, this line of thought leaves us with Palutena, Medusa, Dark Pit, Hades, and Viridi. However Viridi is notable as one of characters that you never fight in KI:U, and I read somewhere - was that TVTropes? - speculation that's because beating up somebody looking like a small girl was considered unreasonable. I agree with this being a plausible reason, and if it's true, it damages her Smash chances too. Then again, Toon Link and Villager are small boys, right? So I won't dismiss Viridi yet, but please keep it in thought.
I will dismiss Hades, but not because of him being the devil archetype or anything. It's more of combination of other factors. Point one, he is a walking spoiler. Not that Sheik is not, but I guess Sakurai will be less willing to spoil his own story. Point two, we have no idea how would we fight. Note that this doesn't mean no moveset potential. It's just that we only meet him in person when he's fed up on souls. Still better then Viridi, I guess. Point three, he would make a better stage or boss if there are any, with
him being too big his body regrowing abilities. Point four, Hades said we'll see each other in 25 years. Could be lying.
Back to Viridi, she could have a very interesting moveset. I envision it as some sort of neverending Reset Bomb lite: plant growing magic, minion summoning, et caetera - but it's just me. Pittoo can do anything Pit can, we've seen Palutena fight in a Smash-adaptable way, and Medusa... Well, here the things start to be interesting. While the way she fought is not something you expect to be very easily convertible to Smash, she's technically Palutena's sister. So cloning potential doesn't stop at Dark Pit!
Anyway, here's my expectations:
- Pit wouldn't probably be revealed that early without some additions from franchise planned.
- We'll see just Dark Pit only if the development team is really rushing after time/money.
- Otherwise, can't see anybody but Palutena taking the second slot.
- If there is a third slot, Pittoo/Medusa/Viridi will take it. Pittoo if Sakurai doesn't care about cloning at all, Medusa/Viridi otherwise, rather Medusa because she's older ingame and outgame.
- The character from the third slot won't necessarily unlock/reveal after Palutena.
- There is a small probability of two of those, probably Pittoo+Medusa, replacing Palutena in case of, for example, somebody going crazy against Pit fighting Palutena because Pit's a devout angel, or because it's a spoiler. Smash being Smash and KI being KI, I find this unlikely.
- In the totally radical case of KI gaining more slots, all bets are off, including Palutena and 3rd slot.
- Sakurai being Sakurai, I'm taking my conscience with a grain of salt.
