Okay, I've just got home, sorry for not answering earlier.
Unfortunately you were not registered here yet when the terrainer was choosen. I know that you're a great terrainer, wish you were working on it from the start
But well, I'm almost perfectly fine with the current one and I don't wan my old terrainer's work to be wasted, think you can understand it.
It's supposed to be flat. Calculating missile distances (skillshot abilities) is easier on it and trees won't block the line of sight. It's fine as flat.
You can import things of course, as long as they're small. As you can see the map is huge already (and since we have over 100.000 lines of vJASS code, even the script has a huge filesize). You can also remake the waterfall, of course and do minor changes. But keep in mind two things when doing them: