About the levels: I`d recomment to keep the game small and concentrate on making the current levels better. Add multilayer floors with details like pipe systems, turbines (in that big thing with the dynamic lights), water / slime, particle effects, colored (maybe dynmic?) fog, ...
Be inspired by this:
http://www.opserver.de/coni_users/web_users/pirvu/au/demo/zips/takeover.zip and especially this
http://www.youtube.com/watch?v=lqwQvacNltE .
Oh, and work on the detail grade of your weapons
.
And if you want to make it really, really, really cool, then be inspired by this:
http://www.youtube.com/watch?v=JcCJrnGWxUA . A lot of work, yes, but it`s the best game of all times to me, so it`s worth it
Are you German? I made a retro review abot it some times ago:
http://www.youtube.com/watch?v=xo7EjhLam10
