My sugestion is, that it should be possible to build villages in the forest or an adjecent into the forest. Then often a unpopulatet landscape is just a huge forest. So it was in the old Europe, it had enough rain and not millions of buffalos. So the early farmer and settlers had to clear the ground for their villages and fields.

So when you build in the Forest it should take some more time but also give you wood as a resource for the buildings and as a trade good.

It should even be possible to build in the forest without clearing, if you want to implement a tree-hugger race.

Also nice would be if their are some buildings already in the forests, as Crystals, towers, bandit hideouts, animal burrows, old ruins from long forgotten civilizations etc.   Fill the Forests with life.

When we are thinking about special building places, keep in mind that their will probably be some mountain folk and some sea diver. (if you remember sids alpha centauri crossfire, the pirate faction had bonuses for sea cities which gave a new dimension tho the card) It would be nice to build a city in the mountains or at the ground of the sea (or a swimming one).

Mountains; they are at the moment impassable. But if you have a mountain chain, which cut through the Continent, will you need passes over the mountains. To secure such a pass should you be able to build a fortress on top of the pass to protect your land behind. Maybe you could also collect duty from trade routes which come along.