Okay. Guys. This is what's going-on.
"Why haven't they announced Mewtwo yet? Why aren't Dark Pit and Game & Watch confirmed on the site yet? Why haven't they shown-us a Dr.Mario costume alt in a pic-of-the-day yet?" Mewtwo is a character that has been in a game before. If anything, they're trying to work-on newer characters before they pick-up and revise older ones ("old" as-in the Melee characters). I'm fairly certain Mewtwo will be in SSB4 at this point though. I mean, he was pretty much the first character "teased" for it. Sakurai said he was considering him sometime after the Villager, Wii Fit, and Mega Man trailer announcements, right? He knew how popular Mewtwo was at the beginning of game development.
Now, moving-on to Dark Pit and the lack of a Dr.Mario costume alt. While there probably isn't a Dr.Mario costume at all, if there IS, Sakurai has probably delayed showing it in a pic-of-the day for ONE reason: in case he runs-out of time. Several characters could still be under development at this time, even though we are close to the Japanese release date. If they run-out of time and are unable to finish the characters, they'll possibly be forced to make clones. This is where Dark Pit and Dr.Mario come-in. It would be much simpler to quickly throw-together a clone if they were unable to finish a character, which is why Dark Pit hasn't been confirmed as a stand-alone character and Dr.Mario hasn't been shown-off as a costume yet. This is why I believe these two characters were used in that "leak" that happened recently, because someone else used the same logic (or they were too lazy to throw-together more character pictures). If this leak is confirmed to be true, we might've lost Mewtwo forever, unless DLC is made for him. If that were his only option of being in the game, I'd take the DLC.
Game & Watch falls-under the last-second category as well. I mean...he's basically a still 2D image. Not that hard to make. He will probably still be in the game, but if any other characters can be completed, he could fall prey to the great subspace abyss...
Well...let's just hope this leak isn't true. A new character reveal soon would REALLY help deconfirm it.