Scenario 1: Deep Dive
Roxas climbs up the Memory Skyscraper and thrusts a Keyblade to Riku, who uses it to fend off Heartless and then face off with Roxas (to be utterly crushed).
Scenario 2: The World That Never Was
Riku lends Kairi a Keyblade to help him fend off the Heartless.
Obvious link here : Keyblade transfering.
This reveals something about the Keyblade's nature : you don't have to be chosen by the Keyblade to be able to weild one, when I used to think only Keyblade Chosen Ones had this ability.
Same with Jack Sparrow, who wields Sora's momentarily.
One thing that bugs me though : where did Riku get the Keyblade given to Kairi, when he didn't get his own until being turned back from his Ansem form into his own body ?
Roxas was chosen by the keyblade and was able to pass one of HIS own to Riku.
Sora was chosen by the keyblade and was able to lend HIS own to Jack.
The only Keyblade wielder left is the King, who (If I remember right) had lost contact with Riku for a long while, to be reunited with him at the tower before the Door to Kingdom Hearts.
Post your thoughs and/ or theories and flame me if this has been posted before.