Apparently there's a Japanese interview where Miyazaki mentions there will be a new, additional upgrade system of some sort to increase player character power specific to the DLC (the comparison given was Sekiro's attack power system).
It's an interesting idea. It lets them scale enemy damage to be roughly equivalent to Haligtree/CFA while potentially increasing enemy health, giving even characters who hit the effective damage cap a reason to explore the DLC area and complete side objectives before beelining the main bosses.
If the google translate version of the interview I heard was correct it seems the upgrades will only take effect within the DLC zone, so that avoids the knock-on effects of, say, increasing the highest possible weapon upgrade level and trivializing the main game.