Regardless of whether it’s officially cannon or not (yes, I know, it’s not), I think the events in Opposing Force are very well thought-out and fit extremely nicely with HL1. I love how the two games overlap, yet Opposing Force continues beyond where HL1 ends, making a smooth transition between the HL1 world and the post-HL1 world, specifically Race X.
The Race X aliens were incredibly well-designed and I really love them, except for the Voltigore (is that how you write it?), I hated those big annoying horse-hippo-antlion hybrids. I didn’t like the idea Black Ops honestly since it doesn’t make sense (why not send in a third squad to silence the Black Ops? Or a fourth, etc…), but I see how the grunts needed another human enemy to make things balanced…
As for Blue Shift, nothing really happened that alters anything (no new enemies and no interesting events, other than Barney escaping). I didn’t really enjoy the game, I especially hated the Dr. Rosenberg crap and collecting those stupid batteries.
I love the idea of playing through the eyes of someone else, re-living the same events from a different perspective.
As for HL2, I can’t imagine playing as a Combine soldier, since they don’t really seem to have a story, though someone could make an interesting mod with that idea… Nor can I imagine playing as Alyx, since she’s almost always with Gordon. The only imaginable alternative story could be Barney, and what happens with him while he’s away from Gordon, or a citizen, but damn, those guys are boring…
I personally would love to see Race X in HL2, oh, and I would also love to see the regular Xen aliens in HL2, while we’re at it. Where the hell did those go?? I mean seriously, why did Valve ditch the Houndeye, Bullsquid, Gargantua, Tentacle, and all the rest (and the Vortiguants, who are no longer enemies), for just 30 different kinds of zombies? I mean the zombies are amazing, but the rest was as well…