I actually posted this idea on the forum last year (http://www.inkscapeforum.com/viewtopic.php?f=28&t=304&p=1693) and then being without a PC with Inkscape for almost 7 months meant I never got around to posting here for developers opinions.
I believe that most artists, when inking a drawing, will ink a line with many strokes to accurately control the shape of the lines.
The problem at the moment is that multiple strokes create multiple shapes. You can hold SHIFT so they only form one shape, but this is awkward when you're using it most of the time. So I would like to see a toggle on the Tool Control Bar that allows for union without holding SHIFT.
Likewise I would also like to see a toggle that has the opposite effect and performs a difference. This toggle obviously conflicts with the new Eraser tool so I'll let you guys debate which is more intuitive or less cumbersome (I assume the erase tool will share the exact same tool controls as the calligraphy tool)
This is a simple screenshot of what I'm talking about - http://www.microugly.com/images/calligraphy-union-difference.png