I always thought the Cardassians had the potential to be a major 1st power threat even without the Dominion. It's just that the Fed had too much of a headstart.
The Cardassians had the infastructure, organized military, scientific development and industry.
If its size were larger and had more ship power and technology, (and resources) they could have easily had taken on the entire Federation, simply because of their organized structure.
But as power after the war, they're finished. Their entire society will probably be reformed- no more empire for them.