It's symbolic, and it doesn't mean nothing.
And people seem to have the knight mixed up.
Terra picks up the Kingdom Key.
Aqua picks up the IKK.
And Ven picks up the Way to Dawn.
ok, i was gonna post that, no, im pretty sure it was Ven who picked up Way to Dawn, not Aqua, cuz the person who picked it up was holding the keyblade backhand, and only Ven does that (as far as we know). but, i decided to read the posts in case someone already said that. apparently, Audo did. u beat me to it! :glaredont take that in a bad way. just meant to be said in a friendly way. ok?)
anyways, so ya, ii dont think there is any significance as to who picks up which keyblade in the trailer, as they dont even use them. VAT use their own keyblades during the fight as far as i could see.