Prior to the D3/D300 I could have thought of a few things. But now I think Nikon have nothing missing. Granted for wildlife I'd like a camera with the D300's crop, but the D3's frame-rate and noise-quality, but no-one does that at the moment (and I suspect that Nikon won't yet be able to either, as compressing the photo-sites onto a DX-size will adversely affect noise).
As far as what I would like - only one thing, but it's the one area where I., a Nikon owner, has real Canon lens-lust! That's Canon's MP-E 65mm macro. I really wish Nikon made something like that! Granted I can go 5x lifesize on the Nikon with two lenses, reverse mounting, sticky-back plastic and double-sided sticky tape (plus a responsible adult to help), but the MP-E 65mm is such a really nice package!