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