I feel they need to take the DS further. Nintendo say time and time again that they want to revolutionise the way you play games - unless the next Gameboy is anything substantially different: apart from a snazzy new graphics chip and control means, then I feel that it's not the way to go, not yet anyway.
Essentially the DS isn't the new GBA in my eyes, I feel that the Gameboy has to lie low for a while. What it is, I feel ties in with what Nintendo continuously say: "It's our new handheld".
The Gameboy is a generic handheld gaming device, I feel the DS is something different, although the two do share their similarities. If a GB is released it'll be a standard gaming machine that has aesthetic and perhaps a few different means of playing. 2005? I certainly hope not; it'll be worse than Microsoft's infestation of Xbox 2 so soon!