Here's a few theories me and some of the people in my anime club came up with about the secret trailer:
My favorite theory is that the 'Keyblade Forest' was sort of a memorial to the weilders of the past. The Keyblade is a timeless weapon that passes from one owner to the next. When a weilder dies, the Keyblade moves to the next potential master and the previous owner's Keyblade of choice is put into the ground in the Keyblade Forest.
The three Keyblades in the ground that most of us agree are Sora's, Riku's and Mickey's, could be a symbol that they've passed on or given up thier titles, but that would really suck. Or it could be a symbol that someone is out to get them.
There is the possiblity that the 'chasers' aren't enemies at all. Perhaps Sora and Riku are two of those figures.
It could also be that the chaser's are out to get the Keyblade for themselves and are going to take the Keyblades in the ground.
Or that they are the next 3 Keyblade weilders, or like the Dark evil sides of our heroes.
Who knows? I think that this was just like the ASAS and Deep Dive trailers, they have something to do with a part of the story, but we will never actually see that scene in the game. You know, Riku and Roxas fought in Deep Dive, but they never showed it in KH2.