I know I might be repeating myself but considering that even the official devs are releasing their campaigns for free on the workshop, I seriously doubt that they’ll try to sell us campaigns/scenarios as DLC.
Civs were the main selling points for DLCs in the past and still are in my opinion. Architecture sets and regional unit skins might work too considering that with those, the civs we already have feel as new civs e.g. Redkirby’s Viking architecture or my Independent Architecture Collection 
HD’s civ system was not ideal considering that it didn’t give you access to all civs you own if you did not own any single DLC, but it still allowed you to try out Premium content for free which not every game does. It’s not something unfixable though considering that Total War has a better system on this regard if I’m not wrong.
I won’t buy a “Challenge” DLC either to be honest.