The man in black coat is Ansem the Wise after KH2.
The new boss might be Master Xehanort's nobody, since he DID lose his body. Though he didn't become a heartless, his body lost its heart and vanished. Unlike Ventus' and Kairi's bodies, when their hearts were taken out of their bodies.
Coded never interested me. *SPOILER ALERT* Basically Coded ends with the KH2 ending being explained. The letter is narrated by Mickey. You get to know that Dark Side is Sora's heartless and that there are many people suffering. The suffering part you get better by looking at Birth by Sleep's secret ending. So all of the three endings: KH2's, Coded's, BbS's endings lead to KHIII. *SPOILERS END*
Though I am waiting for Re:Coded to bring something new storywise.
@NOISE: Yeah, but it is like Square to add something new to remakes. Re:CoM had Zexion as a boss, Final Mix had Xemnas as a boss, Final Mix+ had Terra's Sentiment... Also what made me think the 3DS one is KH1 remake is that the logo is same as KH1. Only with "3D" added.
I actually wrote a long post, but I lost the text, because of a lag.
So long story short, This is what I think would be the value for remaking the first KH to the 3DS:
-Improved graphics, glitches, etc.(kh1 is a really old game)
-3d depth experience
-Riku mode(one screenshot views Riku being played?)
-Things to connect KH1 to the newer titles(twilight thorn), because back in the day the kh series wasnt as expanded