I see the future of Cardassia in this way.
Their territory will be occupied for some time by Romulans and Federation.The Klingons will stay for much longer time in thier occupy zones.
The Federation is likely going to send help to Cardassians in medical and food supplies and try to prevent any conflicts between Romulans,Klingons and Cardassians.The federation promotes peace and cooperation so they will assist Cardassians in making new democratic government.It is very likely that Federation will not allow Cardassians to rebuild their military leaving them just enough Galors and Hideki vessels to defend their territory from pirate raids.There also may be a constant trade route between Cardassians and Feds.
The Romulans will probably leave their part of Cardassian space back to them since Romulan empire is very far from Cardassia and empire needs to be defended.As far i know Cardassian space is poor in resources so Romulans may not see any profit in holding it any longer.The relationship between Cardassians and Romulans will probably be neutral in future times after occupation.
The Feds and Romulans may be gone very soon but Kingons wont.I think that they will stay much longer in zones they occupied to take any resources from it.Klingon empire is big and it needs resources to rebuild fleet witch is lost during Dominion war.When all possible resources are taken they will eventually leave territory back to Cardassians.
The Cardassians will very likely form a democratic government under influence and example from Federation.After rebuilding Cardassia prime and other planets their industry and society will probably grow.When occupation ends, they may start building more advanced vessels and retake some territory from Breen.If all goes well Cardassians may become a fearsome power again in next 20 or 30 years.
But if they decline cooperation and start plotting a revenge against Fed and Klingons they may put them selves into much worse situation.
There is also an increasing threat from the borg.If borg attacks Cardassians in this current weak status that may be the end of Cardassian existence.Or if the borg attack hits Romulans,Klingons or Feds thats mean that occupation of Cardassia may end much sooner.
Everything is possible but as a Cardassian fan i am hoping that Cardassian union has learned a lesson and that it will rebuild it self from ashes and become fearsome power again.