I certainly know both those settings have very big fanbases, especially GH, but in the end, they're both traditional settings like Forgotten Realms. Like many other people say, GH fans nowadays mainly just play the setting for the nostalgia. In my opinion, the two settings are a bit to similar to FR and think it's good to have the more interesting settings like Eberron and Dark Sun being made instead.
Considering how badly they trashed FR when they converted it to 4E, it's probably for the best if they leave those settings alone.
I certainly know both those settings have very big fanbases, especially GH, but in the end, they're both traditional settings like Forgotten Realms. Like many other people say, GH fans nowadays mainly just play the setting for the nostalgia. In my opinion, the two settings are a bit to similar to FR and think it's good to have the more interesting settings like Eberron and Dark Sun being made instead.
What is really wrong with this question is the "instead" thing.
Back during 3e I was frustrated by WotC failing to publish more campaign settings than they did. Now they are actually doing what I want, but doing it at a really slow "one setting per year" speed. I'm finding that just as frustrating. Not so much because of the slowness of it, but because it seems to make some fans think that the only way they can get the setting that they love is to somehow convince WotC to avoid publishing any other settings.
Why can't we have Greyhawk and Dragonlance and Eberron and some other new settings too?
But if they absolutely must dust off an old setting to do a 4E version, I vote Al-Qadim. Nothing has been done with it since the 2E days, and those 2E setting books are expensive. And like Dark Sun, there are some elements that might be done more naturally with 4E than in the original. I'm not positive how much, because I got into Al-Qadim late the first go around and missed a lot of it. But I certainly remember the tone and setting being something that I can see 4E doing well--at least the Arabian Nights and Sinbad inspired bits.
I have to admit that as much as I love Greyhawk I have no interest is seeing WOTC spend time on bringing out a 4E setting and to be perfectly honest I could continue on just fine without seeing Dragonlance.
As far DL being redone as a board game -- that shouldn't be too hard as one of the original modules was a board game (not at home so I can't check which one) so it shouldn't be too hard to rework that into a flashier box etc. and remarket.
Neither do I want to see Al Qadim out again (well that's not entirely true -- but not at the expense of anything else).
I love (or loved) al of these settings but I want something new -- or at the very least something a bit different.
If I had to pick something to be reworked for 4th Edition though I would vote for Birthright -- there is enough different there to be appealing to me.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.