Firstly, there has been a Nintendo-Square deal of sorts, FF:CC...
The only 'other' thing I can 'think' of is, another Final Fantasy for the GC, maybe FF 12. Or Dragon Quest 8, maybe the next installment of MGS, MGS3, a remake of MGS1 with updated graphics, maybe a remake of FF 7... A proper 3D, RPG Pokemon game with online options...
Well yeah... Those are the kind of games that would probably make GC go 'boom'
...
Whatever it is, it must be related to the money gained from the Rare deal. So they could be buying exclusive content for the GC, or maybe a certain company, maybe capcom...
Then again, best to wait and see what this 'announcement' is to see if it's blown out of proportion or not...