In Europe, after Open Skies kicks in in 97 you can base anywhere in the EU (except other countries' overseas territories), and EU gets larger over time.
Elsewhere in the world, you can base in your countries' overseas territories (Puerto Rico, Saipan, etc. for the US)
That's currently the only possibilities. However there's an ongoing discussion about the playability / interest of Africa, and the possibility of creating a Common Skies there (excluding larger countries like S-A, Egypt, Algeria and Morocco), and more generally for every continent's minors.