As expected, an expansion for HalfLife 2 has been in the works, and one can play as Alyx. Now if they would hurry up and finish it off soon...
In the latest issue of PC Gamer it says that the Alyx expansion pack has been confirmed by valve. It also shows some possible features. The 3 that have been confirmed are Dog (Probably npc), Alyx (playable) and the Alyx gun (pistol+smg+sniper)