Birth by Sleep Axel's somebody

Maybe. 50/50 chance it will happen is what i think

 

BBS takes place 10 years before KH1.
If we see Axel or Saix, they'll be much younger. That doesn't mean they don't appear though...

Saix is number 7, and Axel is number 8. As it has alredy been pointed out, they are first to have joined the Organization, after the original 6. This gives them some importance. Axel is also important for obvious reasons, and Saix is 2nd in command or something among those lines.

***SPOILER*** (Written in white)
Also, 358/2 Days does point out that they knew each other. Saix knows Axel's original name.
***END OF SPOILER***

They are both more important than, say, Luxord (*gets shot by Luxord fans*).

But yeah, until the games are released, we can have as much theories as we want.
Name-changing obsessions, odd-condition-created Nobodies, gender-swapped Nobodies, Unbirths, Hearts, Bodies and Souls mixing wildly, all make room for all imaginable theories.