Hi, If you want it in C or C++ then I would vote for QT since it is very well documented and powerful. Otherwise I also think a scripting language is a good choice though I have no experience with it.
Independent of the programming language I have to following idea: Create a pool of available input,output and filter nodes displayed in a tree structure as e.g. known from kcontrol. These nodes can be dropped into a big panel. In the panel the nodes are assembled to a processing graph and modifications can be nicely done with the mouse. (much like the new internal processing structure) A preview window would be nice of course ;-) I have some experience with gui development in QT but cannot contribute much at the moment because of lack of time, sorry. Cheers, Georg