As far as I can see, there is no internal 32GB EMMC available, so flashing a vanilla N900 image on Neo900 may not be possible.

And this is another question: did you have any plans on this topic? Maybe an internal microsd card, so it could even be changed in the future?

Sorry to be so skeptic, my intention is to be constructive. The impression I got as of now is "let's reshape the board and add what's missing", when IMO it should be the other way around, specially if you want to keep it 100% compatible: having the basic shape in mind, try to add the missing stuff (camera and internal storage, for example) to see, among other things, if it is even possible, and then reshape the schematics to fit on the N900 with all board "peripherals" in the right place.

Again, please take it as constructive criticism!