N900, waterproof
USB mouse, waterproof
USB keyboard, large enough to type comfortably, with flat keys, foldable, waterproof, not flexible
LCD screen, large, for watching movies (connect to N900 through TV-out?)
E-paper screen, black-and-white, flexible, rollable, foldable, for reading without wasting energy
Solar charger and hand crank charger (not to mention wall charger and USB cable)
USB/{wired LAN} connector in order to connect N900 to wired LAN
Optical 8x zoom, just in case
And camera shutter can be replaced with fish-eye lens

This all fits into a laptop bag (except for LCD screen) in such a way that you don't have to take it out for airport screening
I'd love to see expandable RAM; I agree.

About the idea of super-computer: is it feasible to have large cube in backpack which would include multiple core 10GHz CPU, 10GB RAM, 1TB memory (for you library of favorite movies/books; for your lifelong collection of photographs; for all the documents you have written; etc) and ssh into it through USB cable which is inside the backpack's strap when your N900 doesn't have enough processing power for a task (like encryption cracking; I can't imagine anything else). Something like Mac mini, just a processing power cube with lots of memory. And it will have to be flash memory, not hard drive, because it will be used on the go.

A dream

I need an optimised Linux for N900. No gnome folders. No co-existence of Qt and GTK and Python and Perl and XUL. No /cd-rom folders. I vote in favor of Minix-like minimalistic XUL-based OS, which cannot run any binaries, where applications are written in XUL and interface is uniform, where window manager is in XUL, too, and ultimately hackable by XUL extensions. No segmentation faults. Instead of X Terminal - Javascript workspace. The shell isn't hidden - it's absent. The low-level drivers are in plugins (like Flash) and are the only binary components (except for the bootloader, but it doesn't count).

With the Electrolysis, and Minix-like structure, problem with one hardware component doesn't make the whole OS crash. It just restarts the plugin.

Gecko OS. Closing the top-level XUL is equal to shutdown. With Javascript, you can ask it to reboot into another XUL (equivalent of current command fennec --chrome=).
Of course, it would be possible to make the bootloader show choices on startup: what XUL application you want to run now, the main one (with window manager and multi-tasking) or a smaller one (camera, browser, text editor, book reader, movie player, video editor, email, calendar).

And the OS would be one-user at the core: no root, no limited users. Though, there still might be the profiles, like in Firefox, with separate extensions/applications, separate history, etc. But from all these profiles, you see the same file-system, without /home/user and /home/root.