Well a mission without any enemies whatsoever would be incredibly boring. And at that late stage in the campaign, the mission may become impossible if you already have full command points. Also, Dunharrow is available to both Gondor and Rohan - what would happen if you decided to use the Gondorian army which can't build Rohirrim?
All the ancillary battles throughout the campaigns are supposed to be about building your army, purchasing upgrades, and ranking up troops. The Good Campaign doesn't need a dedicated "build up some rohirrim" mission, as the player should already have a full army by then - trained and upgraded in the lead up to Helm's Deep. That type of build-up mission only works with Isengard because it's the first Evil campaign mission, and you don't have any pre-existing units. So no, i don't think it's going to happen.
I believe Cel had in mind to edit Dunharrow to include Elrond somehow, with the little forging of Anduril bit from the movies, and maybe Aragorn, Legolas and Gimli leaving Dunharrow towards the paths of the dead. That would be doable, but it wouldn't have much of a gameplay purpose.
As for Osgiliath, i'd have to disagree with Gothmog being there. I thought the Osgiliath mission is meant to be before Osgiliath is overrun by Gothmog and his orcs. Otherwise, gondor wouldn't win, and Faramir wouldn't be there as a reinforcement. I believe the movie clip after the mission shows the real Mordor army heading towards Osgiliath (the one that would include Gothmog). Also, i think it'd be odd to have Gothmog reappear in Minas Tirith if he's killed off in Osgiliath. Yes, i know the good heroes reappear, but it still seems silly.