Coming back to Civ5 over a year later, I've come to the conclusion that Civ5 was a game that did not need to be made...at least in its current incarnation. Modded Civ4BTS was the incarnation of 4X perfection.
Does anybody remember how in SimCopter you could import a real city of yours from SimCity 2000 and fly around in that city that you built? What if the Civ5 team had done a similar thing. For example:
*A turn-based panzer-general type game that comes with a pre-set standalone single-player campaign, but that also lets you import stack battles from your Civ4 game, randomly generates a terrain type corresponding to that in-game tile, and let's you fight out that battle with hex-based tactics at a scale that makes sense.
Or even...
*A Pirates! style game that imports your Civ4 save and continues to have that world function on autopilot in the background (as Pirates! in 2005 already pulled off beautifully to a certain extent) while you did RPG-style quests (randomly-generated by parameters in your particular world, like if there were active wars going on) and played out a career...maybe as a pirate, maybe as a sort of mercenary captain of a band of cossacks, maybe as some modern mafia leader, maybe as some international spy...maybe all of the above...
These are some of the directions I'd have loved to see the Civ franchise go in. For me, a Civ game is a Civ game not so much because of 4X, but because the sweep of history in some way affects the gameplay--because the gameplay is, in some way, immersed in a living, breathing world that is progressing with its own novel, emergent evolution around you.
This is why I very much consider the 2005 Pirates! game a civ game. Unlike other games that might try to do a similar thing (play as the captain of a boat, shoot at other boats, make money, etc.), the gameplay is not scripted like a campaign. There are goals, but there is still a wide-open sandbox aspect to it. Want to track down Baron Raymondo? OK. Want to dance with all of the governor's daughters around the entire Spanish Main? OK. Want to hunt for the lost Mesoamerican ruins? OK. Want to take over San Juan for the English? OK. You could literally remake the map of the caribbean, or watch the nations around you do it themselves with their periodic invasion fleets, or mail runners carrying declarations of war (that you could intercept, and thereby change the course of that world's history, if you so desired).
These are some fresh directions that the designers for the Civ franchise should consider in the future--things that try something else than the 4X (which has already been perfected with Civ4) while still maintaining something that other RTS, FPS, or RPG games don't have, and that Civ designers should know well how to create: an unscripted, emergent, historical gameplay environment.