You need plenty of entities (Whithin Reason) to make a game look, feel and flow realistically. Go outside right now and look around, how much is out there? Unless you are in the middle of nowhere or you live on a house floating on the ocean
There will be lots of stuff around.
In some of your hallways, maybe just put some boxes/ barrels, even just a few of those well placed can look a lot nicer
One more tip, change your yellow lights to less, yellow, nothing is pure yellow like that so try a more orangeish-yellow
Thanks for listening to my big rant up there