Improvements are another thing you can use to stack on terrain tiles. FfH uses lots of them for monster lairs etc. So if you want to combine two features, you can have one of them be an improvement instead. Although, it would be destroyed if someone built another improvement on top of it.
I didn't know that they could be generated at the start, may have to look into it some more. I've set most improvements except roads to destroy the features anyway, because there were some bad clipping issues with some of them.