OK, but what could it possibly contain?
First XP: multiplayer, some new stuff
Second XP: scenarios (conquests), some new stuff
Third XP: some new stuff

, but other than that ???? You can't sell an XP with a few new wonders and units, plus some minor tweakings.
No, I think they will let us enjoy C3C for a year or two, patching it from time to time, then a gold edition of civ3 coming in 2004, and finally Civ4 in 2005 or 2006. Even more plausible seems to be a SMAC2 in 2004 or 2005, which together with C3C should keep the civers content for a few years, until a new release in the civ series, somewhere in 2006 or 2007. Just speculating