EA usually breaks their big games-at least recently (BF2142, C&C3)-in PC Gamer. So we'll scratch that out for now. If the game was SimCity 5 (rumored to exist for some time now) or a new Ultima game, then I would expect the announcement to be in PC Gamer.
Ubisoft just had a blowout with their Ubidays event, where they showed off a lot of PC stuff. So let's scratch them off.
Microsoft has some properties that might be cover worthy, the main one being a new Mechwarrior game. Given that the internal team at FASA is a total goner after Shadowrun, this is unlikely but still possible if they farm it out (which they should do, if they are going to do it). CFS and Flight Simulator are big, long-standing franchises but I don't think they are cover material for a magazine.
THQ is hard to read because of the way they do business with developers, but their exposure to "classic" PC franchises like the ones we would expect is very limited.
So that leaves Take Two, Activision, Sierra, and Atari.
Take Two has the right to X-COM, acquired when it bought Firaxis a few years ago. Now, Firaxis may be working on a new Civilization game (that would be cover-worthy, though it sure seems premature after coming to a close on finishing a big expansion for Civ4 in Beyond the Sword). Something coming out of Firaxis would not be a shocker since they are finishing up work on an expansion to Civ4 and haven't announced the next game they are working on. Also, Irrational Games (owned by Take Two) is largely in the same boat, has a team off doing something secret for some time, and rumors have flown repeatedly that they are working on X-COM, including a website reference that was hastily pulled. Odds are that the GFW game is the new X-COM game.
Sierra has a long list of properties they could dredge out, but there isn't the kind of trail to follow there like in the other games. If they are working on a new title from one of their numerous old franchises, then it'll be a surprise-any speculation to that end is just sort of guessing at this point.
Atari has a TON of the old Microprose licenses that they could dig out and resurrect. The question then would be who would be developing the games. I don't see this occurring in Atari's current financial shape unless it was going to a foreign shop, which would sort of make a cover story unlikely.
Activision is extremely cautious with the PC market. The next Wolfenstien game is a good candidate, though-it's no secret that Raven is working on it, but we haven't actually seen anything from it AND Raven has been very quiet in terms of what their next FPS will be since Quake 4 came out. That's another good bet.
So my bet is either the next Wolfenstien or X-COM, with my bet being on the latter. There are some other interesting choices (Falcon 5.0, MS Flight Sim) but they don't match the kind of game that GFW would get an eight page spread.
It's soo Diablo 3, which will be announced at E3.
If that is in the works, it would come out at Blizzcon, that's the way Blizzard does things now.