The only problem with your post is this:
It makes sence cause where playing as military people...
In case you haven't noticed, only a few of the models are actual military. The rest of them are either cops (two of them) or just seemingly ordinary people caught up in the mess. Hence, there's not as much merit to it, despite the fact that it's implied that the military was sent in.
As for the tl;dr version, the game/backstory implies that everyone is in the military. The models available contradict that.
Furthermore, what is the point of a ranking system in a game where you're supposed to work with your team to keep them alive, aka a cooperative game? How would you gain ranks, anyway? The total number of Specimens you've killed and/or by headshots? The total amount of money you've earned? Your survival rate? A point-conversion of any of the above and more? I can't really see how it would be tracked (then again, I was playing something else for several hours, during which my brain melted, so I might be able to later).
Finally, this appears to be a suggestion, and if so, this is in the wrong forum. Also, please keep in mind for future reference that just because another game had it, it doesn't necessarily mean that it'll work in another one.