The Cardassians will never again be a "threat" species. They never really were threatening until the Dominion propped them up. The only other times that there was an "Oh noes the Cardies are here!" vibe was when they showed up at places that were undefended or had the Romulans backing them up. Their ships went down like punks.
Take away the Dominion, and about a billion lives, and you've got nothing. Can't imagine that the Klingons would have left much in the part of the Union that they received. Romulans probably wouldn't be kind, either.