OK, my thoughts:
1) It should be possible to edit and add terrain types. In addition it should be possible to edit and add terrain improvements. This ability to change and add should be almost totally 'Open Ended'!!!
2) It should be possible to 'mine' forests. i.e. it should be possible to build a 'mine like' terrain improvement that allows you to extract more shields than forest alone (to reflect a lumber operation). It should still be possible to get more shields at once from 'cutting down' the forest, but the remaining terrain should be less productive than standard grassland, and any forest replanted in its place should be less productive than a 'normal forest'.
3) Connected to (2). Perhaps there could be different 'forest types'. You could have a 'plantation forest' which is easier to plant, but isn't as productive (unless you 'mine' it) as 'native forest' which is both MORE productive AND also has a chance of spawning bonus, luxury and strategic resources.
4) It should be possible to 'plant' a native forest, but it should take a LOT longer to do than a plantation. Also, native forests-along with mountains, jungles, marsh and hills-should allow you to build a 'National Park' Improvement. This improvement will generate tourism and, therefore, income for your city! A plantation forest could NOT become a 'National Park'!
5) The number of hexes that contain jungle and/or forest should effect how quickly pollution levels rise-at the city, regional AND global level. This could become VERY important when global warming becomes a problem!
Anyway, just some thoughts. It would DEFINITELY make forests, and most importantly NATIVE forests, more worth keeping around!
Yours,
Aussie_Lurker.