All the parts for this already exist - Nokia just needs to design some fancy new hardware for it to sit on.
Personally I think the EIKON interface running on Symbian OS9.1 (or newer) would be fantastic. The netBook was the best device I ever owned, only being let down by the fact they stopped developing it so support for emerging technologies did not exist. I sold mine approx 3 years ago. Everyone who saw it wanted one.
Long battery life, Instant on/off just by opening/closing the lid, keyboard, touchscreen, flash storage = bring it on.
My only concern is that Nokia aren't the company to do this - commercial pressure to release would far outweigh the sort of quality control that we were used to from Psion.