For every broad, interesting topic discussing <game> mechanics, application and theory to inevitably devolve into the same laser-like focused <edition> war.
However, I suppose that if you exchanged the words in < > for other topics of interest, you would basically be describing the vast majority of internet forum dialogue.
So maybe I'm asking for less axes to grind, less jilted lover behavior, less demagoguery on the internet. I'm sure that will work out!
If I can't have that though, how about a core book that has every single class/race/prestige class possible so that people who have a favorite class won't feel left at the altar. It will probably be 5,352 pages and cost a king's ransom but so be it.
Oh, and although I don't personally use them, it seems that adventure writing is extremely important for a large contingent of the player base. I can't recall reading too many adventures in the past 15 years, but I guess its been particularly bad as of late? If that could be good, it would seem to be a swell idea as it appears that its something of a "getting to know you" gateway for a great many people.