it seems that there are a lot of areas on the map which will not allow building placement. As the game has grown and players islands become very busy, it would be great to either just make a load of extra squares available for building placement (many opportunities for this around the island edges) or maybe offer the chance to build a foundation on some of these squares. i am not thinking that you need to lose all of the mountains or forests but a percentage extra (maybe 10% extra squares).
so you could make it more appropriate for higher levels you introduce a new character at LV40 or so. He is Surveyor or Groundworker
The pathway menu could have a "Foundation" item which you can place on one of the available squares to allow a foundation for standard building.
the square should have a cost ... some squares might cost more than others, like near a resource (fish, marble etc).
once you have a foundation on one of these squares you can place any building on it and move stuff on or off of it It becomes like the rest of the playing area.
If you have a suggestion to make this better please add below. If you disagree, please leave some constrictive feedback so I can understand why it is not a good thing or how i can improve the idea.