We probably wouldn't do the campaign with a DLC map, not all of the team members have Age of Charlemagne and it'd restrict the player base.
Anyone can feel free to make maps such as this to help contribute to the mod or whatever. Here's an asset that might help you.
The yellow city icons resemble region capitals. These have 6 building slots, whereas the white city icons only have 4. It may be possible to change this around, so any settlement can have 6 building slots but I didn't have much luck with it when I tested it. So keep this in mind because for example if you wanted Osgiliath on the left side of the Bosphorus (where Constantinople is in-game), then Minas Tirith would presumably go behind it, in a settlement with only 4 slots.
Now to explain my views on the campaign.
Even if the campaign mod tools were 100% perfect and we could do whatever we wanted, we wouldn't start making a campaign yet. We need to create all the units and all the factions before a campaign can be made.
There's no point making a campaign with only 2 factions, because then we'd have to re-make it later on with like 5 factions, then 10, then 20 or whatever. It'd be annoying as hell (since I'd probably be the one making it).
It isn't definite that we're making a campaign by the way. I'd say there's a good chance it'll happen but its not certain.
There's an even smaller chance that the next Total War game will have the mod tools we need for the custom battle side, AND the campaign side.
There's an EVEN SMALLER chance we'll move the campaign to Mount and Blade Bannerlord.
Any of these 3 things (campaign in Attila, campaign in next Total War or campaign in Bannerlord) are possible but they're not definite is what I'm saying.