now that the trolls are hopefully settled some, I got a couple to add, some gimmicky, some practical, some curious why they weren't included with the current gen, nothing just stupid.
-haptic feedback/rumble pack
force feedback might be gimmicky but it adds a certain quality to fps games etc that gets you much more immersed if done correctly.
-ir sensor/transmitter/camera
remote controls, being a remote control, maybe even mini-kinect to track face/head movements etc
-rfid
not sure what it could be used for just yet, but eager to see what people will come up with as they start finding their ways into smartphones.
-socket analog controls
add in controls such as trackball, joystick, or twisty pong controller thing
for all you retro arcade gamers
-second set of shoulder buttons
for all your sony emulation needs etc possibly analog rather than digital ala xbox 360
-docking port
much interest has been shown in tv out or turning it into a "console", remove the constant charging of the pandora battery, tons of cables etc etc.
-sd card storage
multiple sd card storeage into the battery cover plate. I can appreciate the retro feel of swapping sd card "cartridges" in and out of the pandora.
-slotted notched hinge
unless patented by nintendo or similar, different viewing angle preferences. Some like to hold the device differently than you do... obviously YMMV
-backlit keyboard
when typeing by screen brightness alone is just not enough
-x86/arm hybrid chipset
steveaustin brought up this idea... I silently thought it an amazing idea. arm for when you want arm and power efficiency, x86 for when you want power and speed. How much this would increase the cost and power expense... this might be the most pipe dream of the requests.. probably by a lot.
now most of these things I don't think would use much power if you just not enabled when not being used (except where noted) And as far as R&D costs extra hardware costs take a look at
http://www.dynamism.com/ some of the "handhelds" are $1,000+ so... the pandora is still "cheap" and plus this is a dream sheet... but I understand haters gonna hate. I'm not actually expecting any of these, but these would be in my dream handheld along with mostly everything listed in the OP.