A rotating invasion mechanic would be a great way to keep a potential DD3 unique
At its core, the Darkest Dungeon series contains: a home base (hamlet, Inns), a turn system (weeks in DD1, days in kingdoms, regions in DD2), and different environments/regions to traverse (Weald, Foetor, etc). It goes without saying that any third game would include these core gameplay tenets in some fashion.
Brigand VVulf’s raid into the hamlet and the Kingdom’s Inn sieges are a perfect blueprint for how an invasion mechanic could work. Instead of a singular boss fight like VVulf or the campaign long quests and siege timers of Kingdoms, a simpler mechanic in between the two would add a layer of complexity to any third game and be an interesting avenue for new content.
Imagine instead of a long quest line, you receive an alert at the start of your next turn; an enemy force is marching/marauding through a region, en route to your inn/hamlet with murderous intent. This would start a timer similar to an inn siege, within a certain number of turns you must venture into said region and fight a mini boss, or the timer runs out and you fight a difficult battle at your base.
Such a mechanic would add an air of randomness and challenge as the traditional enemies of each region are supplemented by new foes, and the player is forced to balance the coming threat and their own long term planning. As well, such a mechanic would allow for Redhook to introduce new and old enemies to the game easier than an entire region, as only a mini boss and a few units would be needed to create our invasive factions.