If you want to see the trailer before S-E officially shows it at E3 you can do it right now. The video has been leaked.
S-E could remove the trailer at any time, so here you have a download link.
Remember, KH 1.5 HD Remix is a collection including a remastered HD version of Kingdom Hearts Final Mix and KH:Chain of Memories. Furthermore, this pack features HD remastered cinematic scenes from Kingdom Hearts 358/2 Days.
During the end credits of KH 1.5 HD ReMix, screens from Kingdom Hearts II, KH: Birth by Sleep and KH: Coded are showing up so, maybe, S-E, is teasing a possible Kingdom Hearts 2.5 HD ReMix.