I suggested something similar in the past,
The game has performance issues to keep many zombies active at the same time, I think that's the reason the sleepers exist in the first place,
From what I've tested on small poi it might be viable, but beyond that I doubt it. And it may cause the zombies to destroy objects in the rooms.
Roaming zombies is a great idea if done right, but as said before performance is a big factor when it comes to designing mechanics. It's one of the reasons why bodies disappear after so long whereas before they could be looted. Plus, we do technically get roamers when wilderness spawns wander into POIs when doing quests just to ruin our day.
That's already a feature. It arrived in 1.0 with a limited rollout because they wanted to see how folks reacted, performance, etc. The game will sometimes decide to make a zombie in a volume wander.
You can also make a sleeper volume a "wandering" volume and all of the zombies in that volume will wander, though it comes with some quirks. For instance, the zombies won't start to wander until they get placed and I think if you enter the volume. This generally means a designer is perhaps more likely to use it on a large volume, else players are likely to see the zombies stand up and think the player has awoken them, when in reality they're still unaware of the player.
From a design standpoint, making most zombies roam would make the game easier as they would expose themselves rather than remain concealed behind furniture, etc. Players would get sight lines at longer distances ... hey, as a stealth player, I'd love it as clearing would be a breeze, but then the game might get more boring too.
I agree a 100% and I hate the current approach to zombie placement in POIs but performance is a huge problem for sure; FPS already sinks in big POIs when a lot of zombies are spawned so that would make it worse. Besides, it feels like that ship has sailed.
