As a follow-up to Aftermath (now entitled Episode One), Valve has confirmed that Episode Two has been in development for quite some time ("about as long as Episode One," according to IGN).

No other details are available right now, but it's safe to say that Episode One/Aftermath (whatever you wish to call it) will not be answering all of the questions left open by the Half-Life 2 ending. According to CVG, who will soon post an interview with Episode One designer Robin Walker, the initial release will offer around 5 hours of gameplay and will give more details as to the G-Man's involvement and the fates of the Breen and the Citadels.