I have to agree with Teflon Billy. If a GHCS book was well done, I would want it. I do run my own campaign, but I have found a few things that I like in the FRCS. (One of the most underrated parts of the FRCS was the trade and economic information. It gave a sense of how nations make their money.)
If it is done, I would want it to be done well. Part of the problem would be balancing the detailed approach of FRCS with the amount of freedom GH allows DMs. Based on previous posts, I doubt Gygax would really want to work on such a project. However, maybe he could be brought in as a consultant. (I remember Erik Mona suggesting that it would be great if Gygax could develop some other parts of Oerth if he was uncomfortable working on the Flanaess. I second, third, and fourth that motion.)