has to be a lot less heavier (either make the batt less heavier or the keyboard), look smart, meego has to run better than maemo so should be less buggy for normal use as we waited ages for n900 to be released as it was getting fixed and still got it buggy.
I think it should have a 1.5ghz or 2ghz processor, at least 1gb phsyical ram, 2gb or more would be ideal!
Definetly has to have a capacitive touch screen as thats where all the fun is by having multi touch etc.
Screen size is fine, I think anything above 3.5 is just crazy. But hey if they do it then weight has to be less as the n900 is way too heavy, that is the main thing i dislike about it.
They should also add in a better graphic card in there, default codecs to support mkv,divx etc files for playback.

Im sure there are more things but I just hope they do it right this time, it has to be a kick as$s device for it to win really as im sure everyone has seen what the competition is like now!