they released a side game that only a small percentage of the players could play—which is based on the leaked story of E3

I'm assuming you're referring to Epistle 3 by Laidlaw? In what way is HL:A based on that? I thought HL:A takes place before HL2 (minus the ending scene).

Personally I've come to terms with the fact that we'll probably never get a proper continuation of EP2 years ago. Like I said, I don't expect much from Valve at this point.
If the game comes out and it doesn't have bad writing and features interesting and innovative gameplay I'll be pleasantly surprised.
If the game comes out and it not only has interesting gameplay but also continues to follow the Story that EP2 set up I'll be very pleasantly surprised.
If the game comes out and it's overall OK but they retcon a bunch of stuff and make it so Aperture and the Borealis are no longer relevant, or worse, abandon the storyline for something completely different, then it's time to forget and play some more indie games! :)

Right now I'm mostly interested in what their current Half-Life Project even looks like.