The dev of the zombies mod said that at one point he had hopes to change the game so it is more of a open experince involving more than 1 room. Here is my idea how that could be implemented in a fun and intresting way

Open Mode would start as a normal run, but with much different map generation. First of all, no rooms will have enemies to begin with, instead having zombie pits. Instead of having item rooms, shops, curse rooms, etc., you will have machine rooms which hold 1 machine. one machine room would exist for every machine, or maybe a machine room could hold a few machines of similar types. Boss rooms would block the trap door from being accessed, and would contain no boss.

Powering machines would probably be different, maybe implementing keys to open machine rooms, so you have to think about what room to purchase. Maybe keys could be given every 3 rounds so that players have to be stingy with keys yet it wouldn't be impossible to open up every room.

In order to stop players from leaving a room to despawn zombies, they would have to kill a certain amount of zombies before leaving a room each times. Players would still have to pay a fee to enter new rooms, but it wouldn't increase, staying at a solid 2 or 3 cents so that players don't have to spend a large amount of money just to unlock a new room without any machines.

Of course, this mod seems to be no longer developed, but just in case, I hope the dev sees this discussion so some new life could be breathed into this game. Feel free to build off this idea in the comment section