Pencil doesn't have enough widgets.

Check out cacoo (http://cacoo.com/diagrams) or mockingbird (http://gomockingbird.com/) to get ideas. For example: mouse pointers, loading icons, UML, and other cool stuff. It'd be nice if Pencil had those things too!

Another one is Lumzy (lumzy.com/app). Add pie charts!

And other mock types of charts.

Here's another nice one for inspiration: https://moqups.com/#!/