Keep the weight down to 40 lbs (or less).
Do include speakers, though 60 watts per side is a definite overkill for a portable instrument.

No, I don't need a fully weighted keyboard - let those who do use a dedicated controller, and 88 keys is probably too much. A nice big screen visible in sunlight is highly desirable. To get ahead of the competition, provide a breath controller input like DX7's, so that a player can add expression to reproduction of wind instrument sounds (saxes, clarinets, flutes, etc.)

256 note polyphony with loads of wave ROM will be very useful, as will be the ability to load externally generated samples.

mLan port will be great, and so will be the addition of a high-capacity non-volatile storage media, like SD or Compact Flash cards. Also the ability to play MP3 files.

Studio musicians are not the main market for this keyboard - they can use external controllers (or CVP309), and additional sound sources for extra polyphony. A well-rounded PORTABLE instrument will be primarily targeting all the one man band and small combo performers, who travel between their gigs, and need to quickly set up/break down, and carry their own gear around.