Kingdom Hearts 358/2 Days

The trailer starts from the scene where re:COM ended, afterwards the words "The other side attached to Sora still sleeps" and "The mind comes back whenever they touch." The scene then moves to Twilight Town, where Roxas, and Axel are sitting on the edge, as seen in KH2FM+.

Soon the trailer switches to Roxas being invited to the Organization XIII, and introduced to them, which soon cuts out to another scene of Roxas joining the Organization for the first time. Soon the scene then switches to the King talking about Axel, and Saix with Roxas pulling out his keyblade, moving into gameplay.

The gameplay features Roxas fighting with other Organization Members in a multiplayer game. This one confirms the allowance of being able to play as Roxas, Axel, Saix and Xigbar.

The scene then switches to Xemnas talking towards other Organization members. Xemnas explains that a 14th member has joined the Organization, and the 14th member appears to be a female, wearing a black hood. Soon the title comes up as "Kingdom Hearts 358/2 Days"

You can then hear Roxas in the background saying "I will disappear from this world in 151 days"

Cow's Observation:
KH:358/2 Days is an odd title as you could tell, but if you think about the story's timeline it might not be!

Consider this: KH2 takes place 1 year after KH1 and Roxas' Story starts 7 days prior to Sora's(as far as canon goes it was 7 days). 365 days- 7 days is 358 days.

So with that in mind it could be assumed that the story covers Roxas' point of view all the way to when he appears in Kingdom Hearts II. Now what still has not been accounted for is why it is called KH:358/2 Days... The "/2" is still a mystery. Just keep in mind, this is just an observation!