I would be positively appalled if the Cardassians were ever to join the Federation. Oil and water, my friends, oil and water. I am also in agreement that it would be an intriguing development if the Cardassians were able to rebuild and gain allies to the point of serving as a foil for the Federation, rather than an all out enemy. However, I would see their taking this stance a difficult sell for the people. Even most of the non-villainous Cardassians indicated a certain degree not only of xenophobia but a feeling of racial superiority. It would be hard for people who genuinely believed themselves to be better than all other races to accept much in the way of help or camaraderie from others, particularly others with whom they have bitterly clashed in the past.
I have yet to read most of the novels dealing with Cardassia, either pre or post Dominion, but I did read and enjoy A Stitch in Time
a great deal. I think aside from the fact that they lost a huge chunk of their population, and their infrastructure was seriously compromised, they are facing some enormous challenges, similar to ones faced by Japan post WWII after Hiroshima and Nagasaki.
Entire cultures can and do suffer the effects of PTSD on a large scale. I don't really want to get too into Japan's response post WWII simply because I have some very strong views in light of what they did in China, but Cardassia is facing a similar challenge. Their history leads some--rightly or wrongly can be effectively argued either way dependent upon viewpoint--to the conclusion that they somehow deserved what happened to them. They were unabashed aggressors who had no problem taking what they needed in order for their Union and their people to survive. Of course, this was not necessarily a view shared by every Cardassian, but it is almost always the case that just as people usually bow to the will of their governments, they are also judged collectively by what their governments do.
Undoubtedly, there are many Cardassians post-Dominion occupation who realize, at least on some level, that their governmental structure, their status quo, and their aggression in the Alpha Quadrant left them vulnerable to what happened to them in the end. Not only did it allow Dukat to broker a secret treaty but to take full authority and enforce it with the might of the Dominion at his back. While I agree that Dukat's arrogance is largely to blame for the debacle, he could never have done it without the governmental infrastructure in place to allow it and the power vacuum left by the destruction of the Obsidian Order.
War guilt, as someone else mentioned, will now factor heavily into the collective Cardassian psyche. They could not rely much on the help of others primarily because they had spit in the eyes of anyone within spitting distance already and cast themselves in a cruel light by what they did to the Bajoran people and planet and in prior wars and skirmishes. The one thing I can't see them doing that other races are more prone to is whining about how it was unfair or how they are victims. It's just not in their nature. I personally think one of the strengths of Cardassian culture is the insistence on dealing with reality as it is, not on wishful thinking or fantasy.
On the opposite end of the spectrum is pride. A proud people loathes having to accept aid from others whom they perceive to be weaker or inferior. However, it is an absolute necessity
in their case. They took far too large of a beating to go it alone without being devoured whole by either the Romulans or the Klingons. I wonder how this will affect their views of orphans, if it will at all. In the past, orphans had no status in society whatsoever, viewed to be as inconsequential as insects. Now that there must be a very large number of them and genetic diversity is a concern, I would think this might change. I would also think being forced to accept help from outsiders could possibly work to give them a different perspective on what it means to be vulnerable, if not helpless.
If they intend to thrive and not just to survive, they are going to have to take a good, hard look at the attitudes and ways of doing things that made them most vulnerable, both on an internal scale and in their relations with other races. I doubt that they will be foolish enough to believe that any help they accept is completely free, but I wouldn't put it beyond them also to take advantage of Federation idealism and generosity and offer back as little in the way of gratitude or back scratching as they can get away with.
I don't think we've seen the last of a militaristic Cardassia, or an aggressive one. I do think they are intelligent and complex enough not to make the same mistakes twice. This was an incredibly long winded way to say that the Cardassians are the most interesting Trek race to me. The mere fact that I would be drawn into such a discussion and go into such depth speaks volumes to that fact. I simply cannot see having a similar discussion about Klingons or the Borg. Great topic, Thor Damar!