What about an updated version of Psion's EPOC? :p Just joking... Symbian is all but forgotten nowadays, and it does not make sense in trying to upgrade/update it.
Perhaps my memories are too fond, but I don't agree that it wouldn't make sense to update it. It is/was bloody great OS for the purpose. It was lightweight and did what it needed to do quite well. Lets not forget that Psion 3 (and even 5 to some extent) ran very well on 2 AA batteries.
All the extras coming with "more full featured" OS will consume power.
I now wonder if there really is a market for a real reboot of any kind of old Psion (or whatever) hardware, but with touch-screen LCD (readable in sunlight) as well as long battery power. Should you need colour, just cast your screen to a compatible screen and continue working.
I'd say so. Monochrome is quite enough for normal PDA type tasks and terminal emulation. If you had colour capability to external screen either wirelessly or via HDMI then that could embiggen the market even more I guess.