Need help with a plugin function, please.

As the pictures illustrate, I am trying to figure out a way to make a natural slope from the wall. A natural grade of some sort that will be going into a basin. Eventually, all 4 walls would do that or something to that effect.

The second step I know how to achieve with WorldEdit. Again, I want the slope but I'll be working with a larger area, so doing it by hand would be difficult and overly time consuming.

I found the smooth function, but I'm not sure if it will work for what I want. Any ideas or plugins or any different approach would be appreciated very much!

Thanks!

http://i.imgur.com/8rZE2.jpg - What I'll be working with

http://i.imgur.com/1jXJV.jpg - Second step in the process - At least a 200 x 200 x 75 area is what I'm looking for.

http://i.imgur.com/RtATf.jpg - A rough drawing of what I am trying to achieve