I honestly see Shepard coming back as the G-Man's trump card against Gordon... one last player to keep Gordon from screwing everything up. But that's mainly wishful thinking.
To be fair, there are two things that connect Blue Shift to Half Life 2.
-The teleporter that Dr. Kleiner made: The scientists who escaped from Black Mesa would have known how to build the damn thing, but it took them years to get it up and running because they were wanted men and had essentially steal parts from the Combine to make it work. It's really not the kind of thing you develop on your own without millions of dollars to spend on experimentation, so it's highly likely that this is indeed the course of events that happened.
- Barney himself: The only guard who had any real means of escape was the guard who protected the scientists at the teleporter. Anyone else in the facility would have died against the ENTIRE ARMY pouring out of the portal (or the military if the events of Opposing Force happened). Since we know that is was Calhoun there, it stands to reason that he was Barney, since he explicitly knew Gordon.
Honestly though, the events of Blue Shift are only important in that they point out how much a badass Barney is and how the survivors got out. If those details don't matter to you, then it's not really important. Besides... what we REALLY want is for them make Half-Life 2: Episode 3 or Half-Life 3. It's pretty clear they've been working on it in some regards... otherwise they wouldn't have bothered with the Borealis easter egg in Portal 2.
A few things.
Firstly, Blue Shift Barney's group was not the only people to survive and escape, we know that because Eli made it out too. Of course, his survival is implied to be due to intervention from the G-man, but that doesn't mean other people couldn't have escaped as well. Maybe the G-man plucked them to safety too, who knows? Oh, also, Magnusson from Episode 2.
Following from that, the teleporter technology might not have come from Blue Shift's survivors, if other people survived too. Eli, for instance, obviously knows about teleportation. We don't know where Kleiner was during the Black Mesa incident (though some people think he's the guy who operates the portal to Xen, near the end of the game), so we don't know how he survived. But there's another teleportation scientist right there.
Additionally, the scientists from Blue Shift (Rosenberg etc) have disappeared. Seems like if Valve wanted to imply the HL2 teleporter was based on the technology from Blue Shift, they could have easily used Rosenberg there, either by making him a character or dropping his name somewhere, like in the laboratory bulletin board.
Finally, I would love to see Shepard show up as the G-man's trump card against Freeman, and it would logically follow, since Shepard's the only one who's shown as much badassery as Freeman, if not more. (Barney doesn't count, he's already an NPC) But there are problems there too. If you make Shepard an antagonist, that would be pretty jarring for Opposing Force players. I, for one, don't like the idea of my old hero turning into the bad guy. This is especially relevant since Shepard was a silent protagonist like Freeman was. Barney was given a basically bland happy-go-lucky personality, so it wasn't too bad when he became an NPC, but an antagonist Shepard would need a deeper personality than that. Is he helping the G-man because he was threatened and has no choice? Was he tricked into believing the G-man's intentions are good? Is he just blindly following orders or bludgeoning his way through this dystopian future, like Freeman is? And how do you convey this in a game? Maybe have Shepard betray the G-man halfway through and join your side, that could work. Then it'd make more sense for him to infodump like some other characters have done over the course of the series.
Oh, and I know this is probably overthinking everything, but hey, I'm a Half-Life fan. This is fun.
Added at: 12:59
Oh, by the way, Phil, if you're enjoying HL2 then you might want to look at webcomic called
Concerned: The Half-Life and Death of Gordon Frohman. It goes into the exploding barrel issue.
Some HL2 spoilers though, so you may want to finish the game first.