Okay, so if Firaxis
were to consider another XP for C3, which they're not as far as I know, it would be at the beta testing stage by about mid-2005 (judging by above estimates). Civ4 on the other hand, would be out sometime in 2006 (if they start now, which apparently they are). I don't know, 2006 just seems a little soon for
a whole new game (i.e. new engine, new bugs, etc.).
People seem pretty sure that Firaxis has its hands too full with other games (Civ4 included) right now. That may be true, but all I can say is that Civ3 and its expansions have done quite well hell, even I bought C3C after all my scepticism (I dont regret it BTW a marked improvement), a patch is still in the works and the civ community cant stop asking for more
features. It doesnt look like the Civ3 story is over just yet.
I guess it comes down to this: whos making the decisions and which will give returns within the shortest amount of time, a C3 XP or Civ4.
C3C addressed a lot of bugs and fixes and also added quite a few new features.
Civ4 will probably be on a new engine. The question is, what can Civ4 add that Civ3 cant. Sure, there are the advantages of a new engine with fewer limitations but what is really left to add to the original concept that cant just be added in another C3 XP? A more advanced graphics engine, faster play using a more efficient engine (which as I understand it would probably allow for more features without consuming as much CPU time as Civ3s engine), enhanced internet play, uh
oh, right, a scripting language (although this ones unlikely since this wasnt added into Civ3 when it was possible to do so) and maybe enhanced combat using a battle view or something. Thats all that comes to mind without changing the game too much.
Consider this: how much more can you elaborate on the original concept without changing the nature of the game --and alienating a large part of your audience in the process? In other words, what can Civ4 have that Civ3 cant (aside from what I mentioned above)?
Another thing to consider that will determine Civ4s popularity is how advanced the play and graphics are; if the requirements are too high, part of Civ4s potential audience will be lost just as part of Civ3s audience was lost for the same reason.
It also depends on how long Civ4 will take to be released. Some people may not want to wait.
Personally, the announcement that Civ4 was under development was a real shock. At the time, I was pretty certain that Civ3 was the end of the line or at least the last release for a while (in part, because I figured there was nothing else that would justify a new CIV
except maybe Events

).
Here's what a Firaxis spokesman said about Civ4:
We are in the early planning stages of Civilization IV ... truly the very beginning stages."
"Based on sales of Civ III: Conquests, clearly, people continue to want more Civilization. As long as people continue to want more, we'll deliver."
Heres how the people at Apolyton took the news:
http://apolyton.net/forums/showthread.php?threadid=103185
(Who wouldve thought that such ambiguous information could be the cause behind such blatant use of the 'danceparty' smiley.

)
Youll notice that much of the discussion is centred on how they would prefer the effort to be placed into other games.
I'm no expert but I'd say that many of their Civ4 requests are more than feasible using Civ3s engine.