Replace original SSD with OEM SSD or traditional HD
Do you think it is possible to replace the original SSD of the 13" Retina with an OEM one (cheaper, bigger) or at last with a traditional drive?
I like the Retina and I think it fits my needs except for the SSD size.
Yes its' possible to install a larger SSD check this out as one possibility: OWC
As to putting in a HD instead, No, the system is currently not designed to support it.
I think why they made the space is for future Helium filled HD's that are coming out mid year next year which are still smaller (5mm), run on less power and much cooler than what we have today (1 & 2GB). Heres some info on them Engadget and on the super small drives WD 5mm HD's