Indeed, there will be a lot of construction workers.
The first thing to know about my campaign is that my players, though experienced and talented role-players, don't know jack about Babylon 5. I think one of them watched 4 or 5 episodes and that's it.
They don't know the story of B3 or B4 so I'll use it as an intro to the setting. Here's the general plan:
The PCs will meet on a flight that will be high jacked by terrorists (probably some form of the Martian resistance). They'll end up diffusing the situation and saving the day and the media will make them kind of folk heroes.
So when the government starts building B3, they'll hire them as "special consultants" for the security of the project. This is purely a political move. They don't expect the party to actually help (they're low level nobodies after all) but it will calm the public back home to know they're watching B3.
There, the players will have a series of encounters and adventure focussing on the security of B3. I'll probably have a telepath, a lurker and an officer or agent so the possibilities are qui wide. I'll try to make them encounter different groups and race through this to familiarise them with the setting.
In the end, obviously, they'll fail and B3 will blow up but they'll be kept in their roles. My players are not dumb. They know the series and the game are called Babylon 5 so they'll probably expect to fail protecting B4 too. But they won't. I'll give them some great menaces but they'll manage to save the station until it comes on line and vanishes taking the group with it.
Two years into the future, they'll be evacuated by Sinclair and Garibaldi coming into the B5 story near the end of season 1, just when it gets interesting.
At that point, I plan to have Delenn and/or Drall become their "mentor". I have a plan for a side story involving Talia and they may be recruited as rangers by Sinclair a few months later if I see that it fits the characters.