I would've done it more simple:
It should 99% of the times (1% when you're doing mirroring on TV, etc) be on 2880*1800
And there should be a slider, that allows me to change the DPI. That includes Icons, Text, etc. Images will look the same no matter what. So you can go to the basic DPI that's found on all other Macs (native res), or incrementally increase the DPI until you get to twice the standard DPI (aka Retina)
This way you can get the screen space you want, while maintaining the hi-res. Simple isn't it?
The Retina MBP basically has exactly that in the display preferences, the only exception being the top end '2880x1800 non-retina scaled' option.