I'd love to have seen a Sovereign-Class ship being destroyed on the frontlines, or at least someone mentioning that, "the Sovereign
had been lost in an ambush".
Better tactics and use of fighters would have been nice, seeing a wing of Akira's opening their forward hanger doors and dozens of starfighters joining the fight.
Would also liked to have seen Starfleet more on its own, without Klingon and Romulan support.
More Romulan classes of ships, rather than just the D'deridex.
Commander T'Rul returning to DS9 to act as liaison (the Romulan equivilant to Martok).
Dealing with civilians/refugees from the fighting, the suffering and loss war brings to people on a far more personal level.
Maybe the return of Natima Lang, in exile on DS9 after being hunted down by the Dominion.