I don't see how Valve couldn't pull off a really good opening for HL3. They don't have
to do a direct follow up of Episode 2's ending. It would be more fair to the characters if they did, but if they want the wow factor that HL2's opening had, they might have to abandon the idea of a direct follow up.
The article mentions the drastic change of setting from HL1 to HL2. They could easily
do the same thing in HL3. You have a clear goal in mind by the end of Episode 2 and it's located in a very distant place in the world. Here's just a real quick idea I brainstormed while reading the article. I'll put spoilers around it in case someone as somehow managed to not play through the whole series by now:
Highlight to read:
Valve could simply start you off flying in a helicopter in the arctic, with a very crestfallen Alyx (with a new outfit suited for the arctic) riding along. Dog can come along too, also refitted for the freezing cold if need be. Hell, even Barney maybe? We haven't seen him since the end of Episode 1. Anyway, Valve could quickly recap the events that unfolded at the end of Episode 2, mainly Eli's death and the nullification of the super portal via the rocket; all with some very emotional dialog from Alyx. They could make the inside of the helicopter feel very "Half Life 2," then as soon as the helicopter lands and you step out in to the freezing arctic wilderness, you're introduced to a completely new canon. A nice, fresh start for the series that doesn't completely abandon Episode 2. Simple but effective.
We don't need anything super fancy for Half Life 3's opening. Just for Valve to artfully execute the opening via one or more of the characters and really convenient timing. If that makes any sense to you guys.