I didn't watch the video but here's some off the cuff advice.
Chex 2 already has a city sky texture.
Find some opensource textures of windows and draw over them with thicker lines and enhance the colors. BOOM, building texture. It's dumb and simple but it goes a long way if you dont have anything else.
Use regular hanger doors on the buildings but put a grey border all the way around them and have the doors slide down.
Use lots of 3d sectors and other types of trickery to have the buildings be consistantly explorable from the inside.
Use some giant walls of flemoid slime and police barriers as artificial barriers to guide the player throughout the level......Dont do the Doom 2 thing where you paint an arrow on the ground or something like that. I think TUCQ has some police barrier textures in it, if you can't find them then shoot Boingo a message.
Have at least one section in the level where the player can walk to the edge of a rooftop and see the rest of the city. It's cool and makes it feel more like a city.
Not every building needs to be a big skyscraper, that's not how real cities work. Put some shops or restaurants in there!
Have A LOT OF ENEMIES in this map. I can't stress enough that big open environments aren't fun if there's not a lot of things to shoot at, and big buildings can also provide some necessary cover for projectile avoidance. PROJECTILE AVOIDANCE is what makes Chex Quest in particular stand out from DOOM, so having lots of situations where the player can dodge bullets and hide behind cover builds adrenaline and keeps them on their toes.
Inside the buildings, you can put regular dumb melee guys.
EDIT:: Ok so I watched the video.
You've got a cool hub thing set up here, if I were making the level I'd have a sort of futuristic train station theme here but if you want to keep your kind of red-carpet industrial feel than I recommend some more signs on the walls. Get/make some more custom textures of informational signs to plaster onto the walls. They don't really need to be actually helpful, just some scenery.
On that note, make some kind of wild logo on the floor in the center of the hub to ground it all together.