Now that we have balance patches every 3 months, I really wish they could tie it up with a new campaign for an already existing civ which doesn’t have one. The campaign can be incorporated in an existing DLC if they don’t want to hand it out for free.
On an unrelated note, I really wish that Historical Battles had more battles. The Franks and Britons have a dedicated campaign, and they also have 2 Historical Battles each. It would be better if Hastings were played as Sicilians with some Frank elements. For example, Donjon and Serjeants are disabled, and TA is the UU. When player rescues Harald Hardrada, he gains access to Donjons and Serjeants. The Elite Serjeant upgrade can be researched in the Donjon itself.
Considering that William Wallace is not considered a campaign by many players (it is a tutorial), we would love to see one or two Celt historical battles.