It can only be a multiple of the original resolution. The only option is 2048x1536 (264 PPI) which equates to ~3.2m pixels which is 4x the pixels it currently has.
If I were to be able to choose any feature of the new MBA this is what I would choose as my number one request. I highly doubt we will see this happen but I sure would be willing to pay a lot of money for an advanced display like this.
I can hope and dream, can't I?