A while ago I was going past a building that had this spline like shaped wall and the top of it wasn't straight or even at an even slope...it was curved. Now, when I was in a design firm, I always looked at a building thought "how would I build that or document that"? Now that I'm teaching and helping design firms implement Revit, I now look at building and think along the same lines, except I also think "how would that be modeled in Revit"?
Well, that's one of the first things I did when I had a free moment to play around with it. Of course I could do an in-place wall, but I really didn't feel like modeling all the layers of a cavity wall. Then a little light bulb flashed around in-place elements...you can turn off their visibility. So, I created an in-place floor (could have been roof, ceiling, etc.) and gave it a unique name so I new what it was being used for. I then created a very thin extrusion for the floor shape and finished off the family. I kept it visible so I could attach the wall to the in-place floor. Once that was accomplished, I edited the in-place floor, went to the Element Properties and un-checked Visible. Finished the family and there it was...a curved wall with a curved top! The attached pics show turning off the visibility and how to actually edit the in-place object once it's turned off. In order to edit the "invisible" object, you can go to the Project Browser and use the Select all Instances.
Turning off Visibility off In-Place Family
Selecting the "Invisible" In-Place Family
Now, another method with in-place families would be to use an in-place family (I used a wall) and make it a Solid Void. Create the shape just like the other method above, except that you have to make sure the Void is tall enough to "cut" the height of the wall. Then you use the Cut Geometry Tool to make the Void cut the Wall and then finish the in-place family.