I recently had a Sharp Zaurus C860, which was pretty good, but not as nice as the Psion 5 I remember so dearly! I've often though about designing a modern version of the 5MX, if I had a spare few million, I'd probably give it a go.
I'd personally rather the revised 5MX be a top end PDA, not a mini X86 laptop, as there's already the OQO or Sony UX50 & others in that space (Although those in the 5MX form factor would be cool too).
Due mainly to cost & battery life, I've put down a spec that is like an updated Zaurus (As that's the current top end Psion 5 esque device), but would be in the Psion 5 form factor.
<B>Form factor:</B> Possibly exactly the same as the 5MX, but it might be cool to be able to fit it in your pocket. The 5MX style and design is great, but maybe scaled down to be more like Zaurus, or even HX4700 + keyboard size.
<B>Screen:</B> 800x480 (as in the Nokia 770) Higher res if feasible (LCD, not OLED, due to price & performance in sunlight), just for fun, what about dual 2.6" VGA screens (Total 1280 * 480)?
<B>Keyboard:</B> As Psion 5mx, but opdated so it doesent stick when you hit the side of a key... used to really annoy me! Zaurus keyboard did not feel as nice, but was more accurate than 5MX.
<B>OS:</B> Linux based option (Something better than Q-topia, may have to develop the OS + optimise some essential programs for use on the hardware). Maybe just Windows mobile if this is not feasible.
<B>Software:</B> If possible, run Open Office (Not sure if it's possible to optimise it to run fast on an Xscale or whatever?). PIM software must synchronise seamlessly with PC/Mac/Linux PIM software. If linux, Firefox & Thunderbird will be easy.
<B>Processor:</B> Latest Xscale processor, or maybe a TI one, whichever performs best & is most energy efficient with a good price. Dunno what AMD Alchemy's are like for PDA's, or if they still exist?
<B>Storage:</B> CF & SDIO slot (Or Dual CF?), plus possibly an internal CF slot so that people can choose what internal storage to have, don't pay for it if you don't want it & upgrade it when a bigger CF card comes out.
<B>Battery:</B> Loved being able to pop into a shop & get some AA's when the battery was on the way out, so keep this functionality, but also have Li-ion/pol battery pack that can fit in the same space as 2 AA's.
<B>Comms:</B> Wi-fi, Bluetooth, maybe IRDA.
<B>Sound:</B> Mic, Speaker, Headphone socket + external Mic input.
<B>GFX acceleration:</B>Maybe Intel 2700G, or why not NVDIA G80 + rucksack with water cooling setup, big fat radiator & 2 car batteries?
Well, those are some of my Ideas, I don't know exactly what's feasible, but I think it would be great if a device similar to this became available at some point.