One possible thought for a game would be taking the Half-Life setting and putting your own characters into it.
Now there's two ways of doing this.
Option one is you're some manner of worker in a massive sprawling underground research facility and one day... well let's say that if there was a giant fan somewhere on the property, the main sewage line must be emptying it's contents right on it. Your next step is survival and eventual escape.
In this version there are four 'classes' of character.
Scientist
Security
Technician
Administrator
Option two is the aftermath of the events at the research facility. The world is a mess as 'portal storms' ravage the landscape as to alien life forms that disrupt the ecosystem. Then an actual invasion happens, resulting in a global war that lasts a mere seven hours before humanity surrenders.
Now the last of humanity are hiding in massive walled-in sections of cities for protection as their evil alien overlords strip the world systematically of it's resources. But there is hope in the form of a Resistance.
Are you with them or just going to roll over and let the Aliens win?
Not sure the 'classes' would vary too much but as they say, it's all in how you play it.
Scientist
Fighter
Technician
A good way to get the feel is to take a look at Frith-Ra's Black Mesa Source walkthrough.
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And here we have special guest Werekitty [You must be registered and logged in to see this link.]
If you want a good Laugh, see the original game with the mental dialogue of the silent protagonist.
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Or see the post-invasion world from the POV of the humans who work for the aliens.
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Or a COPS-like show in that era
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