I like your ideas.

Additional stuff I want to see in the next device:
-More Protocols for the standard IM application.
-A better mediaplayer with more formats (mplayer with more arm optimizations would be nice.) It should play the new mpeg4-stuff(h264 and aac) at full speed.
-External connector to attach an antenna for wifi stuff.
-Builtin GPS solution.
-Better quality camera that should be faced more towards the user, so your face is more centered. (On the N800 your face is in the left side of the picture if your about 40cm away.)

Then concerning the keyboard:
I understand what you say. But I don't know if that is a good solution. You would have to add space for the turn-mechanics. At the moment my N800 just fits in the pockets at my behind. I'd hate if the next model couldn't do that.
There is no way of typing with more than a few fingers on such a small keyboard. So I guess I'd be ok with a slider keyboard. I think I'd type on it like I type on the onscreen keyboard now: with my 2 thumbs.