I really hope so. They could offer them for download on civ3.com (wasn´t it mentioned somewhere that Fireaxis has such plans for the time after the release?)
And I really hope we are able to create our own civs as well
If you mean that the new civs in the expansion pack will have their own architecture, leader face, special units etc, I will buy all the expansions packs they can do.
I sure hope so... and I would say it is almost certain, since this seems as a good source of revenue... maybe they were thinking about this when they decided to have unique civs (and only 16 to start with)