I think it would really help the game feel more mysterious and dangerous if certain types of strategic terrain blocked vision. With forests and swamps being two good examples. The ability to  sit outside a swamp and know what is inside it is somewhat immersion breaking. I think it would be really cool if you had to actually go in and explore the swamps to find out their contents.

This could be accomplished by making forests and swamps block line of site completely, so that you can't see beyond the first layer of trees. You could see the trees blocking your site but not what is behind them.  Alternatively or additionally you could make it so that you have to be directly beside a forest or swamp tile to see what is on it. Units and goodiehuts on these tiles would thus be invisible and shrouded unless you were adjacent to them.

This would really add flavor to forests which right now are just tiles that use up movement. It would also make maps more dark and mysterious, while helping to keep some monster lairs and goodiehuts hidden until later in the game. Of course ZoC would still reveal terrain and this would give players more reason to spread it and clear forests. Traditionally forests are dangerous places that are hard to explore because you never know what is hidden in them. I think Elemental would benefit greatly if it replicated this feeling. Right now forests take up a lot of space but are quite boring and pointless.