Night seems a little too dark and day seems to be also (judging from pics).
So at the moment you have a box, I'm hoping there will be more to your map.
So what I can see here are my suggestions
I would suggest having displacements that are higher than the player that the player cannot get to, so the zombies may drop down from a certain area/areas.
I thought perhaps a graveyard on a hill, you could have a locked gate leading to graveyard but otherwise rest of the graveyard is too high for player to access but zomies can spawn and drop into the game area.
Also having displacements higher than player means you can hint bush areas off by having no-draws or walls/occuluders inside the disps. Allowing you to hint brush your map.
Also you can fit your tree cards on the high displacements and save the game area for the odd few high poly trees.
Here's a sample layout that I expertly threw together with paint
Its basic but if your learning about hint brushing (Like I still am then this will give you an idea of how to hintbrush out door areas, atleast I think).
Its to give areas where zombies can spawn and objects can be hidden in other parts of the map apart from the main game area, where the house and most of the shooting will be going on.
I know its a bit symmetrical but its just an example. All you have to do is subdivide all your displacements, and the ones that have a occuluder or nodraw inside paint the geo so the corners of the nodraw are hidden into the disp geo. and throw a hint brush over the top horizantally above all your vertical hintbrushing.
I would send/make you an outdoor area if your stuck on how to but I'm like halfway across the globe from my broken PC right now
