Valve can't do it, no one can.I guess if
Remember the hugely popular Warcraft III mod where you have a time limit and resources to buy new things, just to level your opponent's base? Yeah, it's a sequel to a game.. within a game.
I say bring it on.
I invested no small amount of time in Warcraft Battle.net playing. I never became very good, but I loved it and played often. I'm sure if I were to install the game right now and log into battle.net, I would be doing nothing else for the rest of the day. Well that and drinking beer, and looking at pictures of Lindsey Lohan's giant bombs.
The battle.net user only known as "IceFrog" was hired in 2009 to help make the game. He was the moderator of the most popular version: Defense of the Ancients: Allstars.
I made mods for Doom II back in the day, but ID never hired me to make their sequel. This could be good and bad, but I'm now interested and I'll most likely buy it.