Form Factor: Folding device with keyboard similar to the Viliv N5 or a slider form factor
Screen: 5.5" 1080p display with stylus support
OS: multiboots Android, Ubuntu, Tizen, MER, Firefox OS, and Sailfish
Processor: Dual Core Arm Cortex a-53 or better
Ram: 2gb or better
Storage: 128gb plus microsd expansion
A device like this probably won't exist for a while and probably not with the form factor I want either. One can dream though

Ok?

ten chars

Untill that day, you can find yourself a N900.

I am too conservative for a multiboot. I would rather have one operating system, and tweak it, until it works-for-me. Right now, Fremantle is the best.
Yes, stylus is a must. Though I would prefer the screen to be scratch-resistant (akin to wrist-watch) and more sensitive to pressure. Contradiction, isn't it?
Screen is fine as it is. Though, I would not be opposed if it was high-resolution, like retina. But then, I would prefer to somehow have two screens: one low-resolution, monochromatic, for the graphical user interface; and another high-resolution, high-color-depth for photographs, camera, movie recording, editing and watching, drawing. This way, you can check time, send SMS, make calls without using as much electricity. I know, I am a grumbler, but if you have to spend a week with your phone without running out of charge, such an option would be handy.
Processor: fine as it is. RAM: 2GB, not more. Storage: 32 GB+option to expand it.
Form factor: preferably slider, but it has to withstand mountain hiking, sea sand, sharp stones, lake water. Overall, it must be more difficult to squash than a brick. Yes, I am paranoid. And extremely attached.
Additions:
magnetometer (compass, and metal finder - if you lost a needle or a tiny screw),
radio transceiver (FM radio, Bluetooth, analog television [imagine how handy it would be in these times, when a lot of people have old analog TV sets, and many TV channels have switched to digital broadcasting; just TV-broadcast a movie from your phone, and you can watch it on any analog-TV display within ten meters]; accepting external antenna for either transmitting or receiving),
TV-out cable (just in case you wish to show your photographs to many people at once),
barometer (weather, or geographical height above the sea level),
Wi-Fi (all its many variations),
microphone with noise-cancelling ability (I am quite tired of observing people yelling into their mobile phones),
speakers and microphone which allow constructing echo-hologram (even if it requires about a minute of CPU-intensive number crunching),
palm-sized paper printer (handy for visiting cards and QR codes)...
[Edit]Camera. Must not forget camera. With flash (maybe, ring of light around the camera). Maybe, capable of moving around, for better panoramas. Maybe, capable of cancelling out the movements of the vehicle. Maybe, higher resolution.
And notification LED.
And both camera and screen must have dynamic range close to than of a human eye, probably slightly higher; about 13-14. Then you will not have to capture multiple standard photographs to get an HDR photograph.[/Edit]
Best wishes.
_________________
Per aspera ad astra...

I am with wikiwide except that I think printing should be a dongle, QR code works for v-card trades between smartphone users.
Also I look forward to when highter power LED projectors will fit inside the phone, perhaps RGB laser scan projection.
As for TV transmitter out I would just want a good unlocked software defined radio with a fast DSP for in/out so I can do Bluetooth, WiFi, GSM, amateur radio, aircraft band, FM radio short range, GPS, FRS/PMR446, etc. There are times where I have in my flight bag a laptop, my N900, an air band HT, and a micro size UHF/VHF amateur radio HT, at least the N900 replaces a phone, GPS, MP3 player, gameboy, camera, voice recorder,, and PDA.
I dont mind something larger like the giant Galaxy tablet phones but I would like a sapphire screen, titanium body and vulcanized rubber bumpers like the Vertu phones have, I suppose some would like the leather and hardwood trim found on some Vertu phones.
I have several watches with clear sapphire crystal, it stays shiny and scratch free even after years of hard use even while the stainless steel shows some history. I always wonder if there was ever a Maemo/N900 type prototype device with a extreme materials case over at Vertu when that was a Nokia property.

You guys have wishes ...

My "dream"device:
a) CalDAV sync working flawlessly
b) CardDAV sync working flawlessly
c) reliable Alarms
d) Alarms working in "off" mode too
e) good GPS performance
f) Video recording stable and fluid (without hacks)
g) voice dial supported
h) fully matured PBAP support
i) reliable BT transfers in both directions between mobile/mobile and mobile/computers

N900 sucks at all of that besides (d) -> i absolutely could live without plastic keyboards and stylus for having an actually reliable travel or otherwise companion.

SGS4 hardware and MAEMO on top of it , that would be a nice dream

Why ? I tried galaxy note 2. It lags sometimes, iy's not only hardware performance. My N9 don't lag a lot, even with no tweaks.
I think these super power hardware are not necessary.
All we need is a software made for a hardware.
A eeepc 701 is faster with archlinux than these last windows 8 crap tablet.
Performance on a quad core phone with 2gigas ram are far from what we know on a computer... Why ? Because you only care in text on the box.

128gigas ? Really ? I don't know what to do with 32gigas +sd on the N900...

My dream phone, well I have 3 ideas.

1) The first is a NOTE with better hardware, features and software

2) The second is like a N97, only with a very skinny (12mm) form, a huge 4.7in display and a convertable. The convertable is where the qwerty is, the qwerty can be removed and other peripherals can be placed inside such as an extended battery, a qwerty of another language, otherwise a (non-sucky) gamepad

3) This is my new one:
A large slate phone (like NOTE) with a thinness of 13mm
Minimal bezel, with a 16:9 display that measures 5.7inches.
The phone is actually folded into two, with a flexible touchscreen on the outside curving about the hinge.
The hinge opens the device to make it one flat touchscreen and doubles the display area.

So it "transforms":
- From a 13mm thick device with a 5.7in 1280 × 720 (258ppi :1.77) display
- To a 6.5mm thin devices with a 7.5in 1440 × 1280 (258ppi :1.125) display

Basically a pocketable tablet. One which goes from a 16:9 aspect ratio to a aspect ratio that is square-er than an iPad, hence more comfortable to grip, hold and rotate.

imo, the N900 without the hardware failures (usb port, etc) and 512 or 1gb ram would have been dope.