Not gonna waste anyone's time so I'll cut to the chase, you read the title, here is what I'm thinking.
1. Add plaguewalls to certain houses or specific large buildings in each zone that contain plague hearts. Thinking 2-3 per building(killing any of them would allow access to the heart, killing the second or third would just open up additional escape routes making it "safer"). Limiting the number of entry and exit points making it much more claustrophobic and dangerous to fight in, as well as funneling the zombies through specific routes with a good chance to cut off and trap the player.
2. The Heart in these buildings should also have the bloater belly of a plague wall in order to be damaged, this forces the player to actually fight the zombies waves spawned.
3. The Heartland plague zombie variants would have to be added, as would quests to get the crafting mod to craft the plague busters, which brings a general difficulty increase with the plague zombies on all kinds of quests be it freak in the kitchen, to sieges, to completing the legacy mission as well as play in general.
A. Increases resources needed per heart kill, making already scarce resources even more important, and works on the resource expenditure side to create scarcity rather than just saying no don't go here it's empty to the player(which is honestly a waste of the beautiful world you guys have made).
B. On the development side it is mostly existing assets that the UDL Team has already developed, be it code, art or quests that could be adapted with limited people hours.
C. It's quite literally a death trap for players, they can expend more resources to make it less deadly, to a point.
D. It would only be a certain number of the hearts, on maps with already limited resources its meant to be an additional strain for when you've normally got things in hand, and are coasting towards the legacy completion..
E. Has a low system resource impact by ramping up difficulty through resources, terrain and the heartland freaks, rather than just spawning more zombies/hearts.
PS. I said red zone, but you know whats red and kinda heavy into the lore of this game? Blood. Blood Zone is my vote ;~) .