The way I see it, there should be THREE main routes to creating borders.
1) Cultural: This is the current method of building cities and culture as a means of growing your border. Perhaps SMAC border styles could be applied to this, so that your borders start larger as soon as you build a city, but get subsequently 'squashed in' if you lack the culture to maintain them in the face of a higher culture city.
2) Military: Outposts and Forts CAN be used to expand your borders IF you have both the troops to occupy them, and the underlying cultural strength to enforce your claim. Culture effects the maximum border size AND the maximum distance from your cultural borders that this can take effect.
3) Diplomatic: An adjunct of 1 and 2, you can give away-or ask for-forts, outposts and cities near to your respective borders, as a means of exchanging the land that lies beneath. It might also be possible, in this method, to select territory via a form of MS Paint system in the diplomacy screen-but it would rely primarily on the exchange of actual structures.
Anyway, hope that all makes sense.
Yours,
Aussie_Lurker.