I don't think Nintendo will release a new DS for at least another year (so earliest would be late 09).
As for the design, I don't think they'll go smaller, the DS Lite is plenty small as it is and some people even complain that the size makes playing games for long periods impossible (hand cramps etc). I'd tend to agree with the folks at IGN here, it'll be larger buttons and larger screens (though that doesn't mean a fatter DS).
And they won't, I repeat WON'T, remove the Slot 2/GBA slot. Simple reason is, as many people have said, rumble feature. Not to mention the new Guitar Hero DS game using the slot for the threds (and with that, the implications for future games to use the GBA slot in a similar fashion). Why would Nintendo completely ruin a new gameplay style by removing the slot (They are all about "new" gameplay designs after all).







