I think that the changes won't be as drastic on the outside for the next-gen Nintendo handheld. The control scheme could be refined maybe (an extra button here or there, the d-pad or analogue stick argument), the screens will be improved with whatever technology has the best price point for Nintendo..small changes on the outside. Whether or not they continue with the dual screens who knows. It's proven popular enough, so I wouldn't be surprised.
Where I think the changes will be more drastic is under the hood. More battery life here, a more robust 3D graphics implentation there, larger storage sizes, etc. Nintendo has shown us with all the other Game Boy revisions that after they do a major hardware change, they generally don't alter the formula too drastically for the next round. We went from monochrome to colour, playing with the unit size along the way .. from colour to mega-colour, and playing with the orientation along the way .. from no backlight to backlight .. from backlight to brilliant backlight .. etc.
It's not like we're going to see three screens and an extra stylus - we as human beings won't change, and won't suddenly be able to handle more on the input side. I just think we'll see some minor hardware refinements, while being given a boost of power under the hood.