Purely Cosmetic, this will be paying for additional paint variations. Time limited or otherwise. And it would be cool if you could even pay to upload your own custom paints or battalion/squad logos for a certain period of time. This is the area I am most interested in.
Greater Access, this may seem to be the same as the Pay to Win section but I am separating the two a bit. This area is is more about access to features that effect gameplay variety more then gameplay advantages. Such as access to different maps and or areas.
A mix of new areas, maps and cosmetic features would be great. Take a look at the LotR Online F2P model. It works and Turbine make a good revenue ( at least the say it works well ) with it.
A big & definite NO for pay2win. It would kill the game before it really launches.