The trait system stands as it is. There will be no new traits added on and there will certainly not be "make your own trait" trait.
We'll have to assume it goes as follows:
A large population isn't something that requires a specific trait because it won't lead to any spectacular advantage on your part. You can certainly cite it somewhere in your population description. There are plenty of places in your profile for you to make mention of it. However as far as the strategy of the game is concerned the population is pretty much irrelevant.
You can translate your high population to one of the traits already in existence. For example, the Battleships! Trait could be altered slightly to specify footsoldiers or something like that. There being more of you may seemingly lead to advantages, but at the same time you need to clothe, feed, and equip all of those people. It sort of evens itself out.
You can find creative ways to apply the different traits. The system is actually more diverse than I thought it was when I first looked at it. Just find little loopholes and manipulate it a bit. I mean a BIT! Nobody in here is going to try and godmode, I'm sure.