They're probably not doing Snake again. Kojima has made it clear that Snake's story is completely over.

I doubt that they'll made a sequel with Snake as the main character. Probably he'll only appear as a support character or just in some quote or reference.

The most probable is that they'll made something with Raiden.

I don't remember where but someone told me that they were preparing some add-on content that'll be two completely new acts in MGS4 with Raiden...

MSG eithout Snake would be weird though... he's the main character and everything. It's like Terminator without terminators.
Maybe there'll be a "new" Snake? Or they could just make another prequel.