The number of villagers a village can sustain is based on the number of doors in the village. The Village mechanics page on the Minecraft Wiki gives you the details but it takes about 3 doors to support one villager.

Anyway, if you assume one or two doors per structure, it still takes a lot of structures to increase the sustainable population of a village. But, doors can look like windows. Of course the window must be two blocks tall but that is generally not a problem.

It’s simple enough to do. Use RPW or other resource pack tool to change the images associated with any door you choose. I use dark oak and acacia doors because I don’t usually use those varieties. You can use transparent images so the door looks like a window but you can see through it.

Remember, it takes more than a door block for the door to count. Again, check the Village mechanics page for details.