I'd go with a Size 3, max. Somethingg along the lines of a Nova or Defiant. I'd make the venue someplace frontier-ish. Maybe dealing with the Breen or some new threat that has popped up politically due to the war.
Best ideas would probably be: 1) Cardassian UNion splinters into warlord factions. These guys are working with Starfleet & the main Cardassian leadership to prevent the warlords from gaining the upper hand, powerwise. 2)Cold war stuff with the Romulans in some place uncontrolled by the original Romulan Treaty -- I used the Draconis Outback & placed it where the Cardies and Rommies could both get to, then played a lot of strange politics -- worlds with populations split during the war due to occupation -- Fed race/Cardies on one, Romulans under Cardie control on another, and the like... 3) The major powers are hurting bad from the war; a lesser power, like the Orions for example, move to gain power in the region. The crew is watching and trying to stop them from grabbing colonies or other non-aligned worlds. Loads of training indigenous guerrillas, etc...
But the best idea..? Combine 'em all.
"War is an ugly thing but not the ugliest of things; the decayed and degraded state of moral and patriotic feelings which thinks that nothing is worth war is much worse. A man who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself."
John Stuart Mill