Your vehicle looks great! I like it pretty much the way it is, so if I were you I wouldn't modify it much. I might suggest the following modifications;
1. I agree with others that xenon headlamps would look good. But in contrast to what Rollin Thunder suggests, I would keep the fog lamps halogen. Xenon fog lamps are tacky. The contrast between the xenon headlamps and halogen fog lamps looks good, and xenon low beams would ruin that contrast. The only exception to this would be if you perhaps did 3000K fog lamps and 6000K headlamps, which would still provide the contrast, but still may not look as good as simply leaving the fog lamps halogen. If you did the reverse and installed xenon fog lamps and still had halogen headlamps that would really look out of place.
2. I might also suggest the high/low mod as well once you get the xenon headlamps in the low beams. Most vehicles that come with xenon headlamps from the factory, if not bi-xenon, keep the low and high beams on when the high beam is selected. Otherwise when your high beams are selected it will turn your xenons off. You could take it a step further and do the high/low/fog lamp mod so they are all on at the same time when you select your high beams. This is a simple and inexpensive modification as well.
3. I would recommend GMT 800 GMC Yukon Denali wheels on your vehicle. I have seen them on GMT 400 GMC Yukon Denalis and they look especially well. They are slightly larger and the shape is similar, but look better for whatever reason. They would still match your vehicle well. It would look slightly updated without looking flashy.
Other than these modifications, I see little room for improvement. You already have a great looking vehicle!