So I've got to ask: Why doesn't it look like a storm water sewer ... at all? Considering some of the amazing architectural achievements gained in this area, I'm surprised you would have such an advanced civilization (there's one government, right?) have such non-functional sewers.
Why is there like ... human stuff in it? Even if the thing is "abandoned" it would still collect storm water. (Maybe look up how these things work?)
Might I suggest taking some notes from current technology? For instance, the (as far as I know) most advanced
storm sewer in Japan.
As far as gameplay goes you could easily have some amazing dark atmosphere with unknown enemies ahead (zombies? awesome). There's also, if done correctly, the effect of chasmophilia (I think that's the term ... that's what google says, in any case) which is, pretty much, the opposite of claustrophobia.
You could also have it to where, you need to do stuff in a timely manner because storm water is collecting in the reservoir shutting you out to areas you were in previously, while there's an ever encroaching need to get out of the facility for fear of the enemies there, but also the water that is collecting in the structure.
It may help you to think about areas of the game more along the lines of gameplay rather than plot, the best games ever made had plot and story that was initially grafted onto the gameplay. (The Legend of Zelda and Half-Life are a couple of games that were crafted this way)
It would allow you to design the entire game then work your story to suit your gameplay. Of course, having an overall idea of what you want to happen is not bad (and can help keep continuity). But what you're showing here so far, (I understand it's one map) is rather mediocre looking as far as gameplay goes. I played HL1 and both of it's add-ons like 8 years ago ... there have been large advancements in gameplay and that's something you should keep in mind (especially because it has nothing to do with technology, but rather level design)
Anyway, good luck and all that jazz.
Oh one more thing. If the game doesn't have zombies in it (like motherf**king dawn of the dead zombies) ... I will be disappointed. Post apocalyptic futures just don't work without some sort of "zombiesque" aspect.
Blame it on Microsoft, God does.