GoNintendo are reporting, based on a tip from a reader, that Square Enix have come to a licencing agreement with Nintendo over a character or characters from one of their series, a character (or characters) that hasn't been used for a very long time. Where this information comes from or even how many characters the licencing deal involved are undisclosed.
This is the best sort of rumour: the sort which has no boundaries. The most obvious solution is either another Mario Sports Mix (although Square Enix developed the original, which would make licencing characters to Nintendo odd) or cameo appearances in the next Super Smash Brothers, which must be in the first stages of development now that Kid Icarus is on shelves.
On the other hand, feel free to let your imagination run wild. One side (half-jokingly, I hope) reckons a new Super Mario RPG could be on the way: and why not?