Let's them finish Episode 3 first :D
But I think it will become epic over epicness, Episode 2 was the shit already
I'm with you, Gabe said that Ep3 will be epic (or the end, I don't remember), also, Valve has a lot of things that they can do with the plot, like [spoiler]a dark Alyx that in the end will try to destroy the Borealis like his father said, and then... well, maybe she die. It will be shock that you have to kill her or something like that. Also, I'm sure that it's very important when on the Ep2 Alyx is being healed by the vortigaunts, and they say something like "it's necessary an human, Gordon, your life is connected to Alyx's now", before G-Man appears. And if Gordon kill her, something could happen with him. But I'm sure that Valve will do something better. [/spoiler]
One more thing. Maybe Portal 2 woudl hints something about Ep3. Like on Portal, with some posters. Who knows. I'm not speaking about HL characters playing cards with Glados, but... well, you know. I think that this year or 2012 Ep3 will be announced. I'm 100% sure.
If this E3 Gabe says "L4D3 announced!!!1" I will kill him.