I really like the idea that a given civ should not have the same traits in every game (it makes no sense for a civ to be seafaring if it's not on the coast, etc.).
I also like the idea that a civ could potentially
change traits during the game, although its worth pointing out that this is not necessarily a consequence of civs being able to have different traits from game to game (i.e., it would be possible to have a system where once you acquire a trait it would be permanent).
But I think this idea needs a lot of careful thought. For one thing, as CIVPhilzilla pointed out, there is a lot of potential for abuse if it isn't done right (in particular, I'm guessing it would be hard to program the AI to be as crafty about trying to get certain traits as human players would be). For another thing, such a system has the potential to introduce a lot of complications to the game (for instance, would the "points" you have towards each trait be displayed anywhere - would the player know them? One more thing to keep track of, if so, but hard to understand why you did or didn't get a trait, if not). Lastly, I'm afraid of too much "rewarding of success."
Let me elaborate on that last concern a little, and maybe others can propose ideas that will reassure me

. In general, in a game like Civ, success is its own reward, i.e., conquering lots of enemy cities or being the first to get new techs are advantages by themselves. In fact, the designers have carefully
reduced these advantages to keep successful civs from growing out of control and dominating the game too easily. Things like corruption and resistance make conquering cities a little harder, techs are more difficult to research if no one else has them yet, and so on. Now, it does make sense, from a "realism" standpoint, that civs that spend a lot of time conquering, or researching, or whatever, would get to be extra good at those things. But my concern is that, if you reward a player who is already successfully conquering neighbors with a "militaristic" trait that makes further conquest easier, or if you reward a player who's ahead in the tech race with a "scientific" trait that makes it easier to stay ahead, then those things work
against the system thats already in place to keep civs from "snowballing" to quick dominance.
So for those reasons, I'm nervous about the ability to acquire new traits, even though I do like the idea in concept. One version of this idea that I would favor would be a simpler system, in which you simply pick which traits you want, rather than having them based on your play style. Perhaps the choice would become available whenever a new age started, or when certain techs were researched. With that system, you could choose to be militaristic when you were preparing for a grand conquest, not become militaristic as a reward for success in that conquest. However, some of the same concerns, especially how well the AI would handle things, still apply to this version.
It's funny, but all of this talk about being able to determine your own traits and change them during the game makes me think it almost might be better to eliminate traits instead

. In Civ 2, you could consider yourself a great seafaring civ if you had a lot of coastal cities and a big navy - there wasn't any game mechanics trying to force certain civs to develop on the coast (and create big navies) by designating them with a "seafaring" trait. Don't get me wrong, I like the traits, they add interesting flavor to the game, but the drawback (as they exist in Civ 3) is that they force certain civs to try to develop in certain ways, and this whole discussion about setting your own traits is really a discussion about how to overcome a problem that wouldn't be there if there weren't traits to begin with! A little ironic, if you ask me
