What will, or Should, be Valves next big Game?

Valve is a very beloved game company that truly expounds on what gaming should be. But after their successful release of the amazing Portal 2 game, what's next? Sure there are rumors of them possibly making a PC based console but is that really their next big project?

When we think of an upcoming Valve game, we think of sequels to the game that are already awesome. Here are some of the big possibilities:

Half - Life 3 (or another episode)

Half - Life is a huge franchise of Valves, possibly their biggest, and it's no wonder that people have been anticipating another game entry for what seems like forever. It is highly rumored that the next game will actually be Half - Life 3, but at this point we don't care if it's even just another episode.

Portal 3
 Portal 2 came out just last year, but people are already demanding for Portal 3. It is bound to happen eventually, Valve made that point clear, but could it be their next big game? Would they want to make the third to quickly finish off the Portal games as a trilogy?

Left 4 Dead 3
 Why not return to the Zombie series? The second game proved to be a great sequel to the first, perhaps the third can make that even better? 

Team Fortress 3
Another game that ends on the third note, could another Team fortress be on its way? I'm not a big fan of this series because it's my least favorite of the Valve games, but could it be coming?

Perhaps a Collision?
How does "Half - Portal: Left 4 Fortress 3" sound? Joking aside, we know that Half - Life and Portal take place within the same universe, so a game like this is not far fetched at all...

Other Possibilities:

• Alien Swarm 2
• Deathmatch 2
• Day of Defeat 2
• Dota 3
• another Counter-Strike? (besides the next one coming in 2012)

What do you think will be Valves next major game? Will it be a Sequel or something completely new and Original? 

UPDATE: Valve is releasing Dota 2 and Counter - Strike: Global Offensive this year, but I speak of what game they haven't announced, the one(s) coming out after 2012 so to speak.