A novel approach to effects.
I've often had the thought that the Jack paradigm of working with audio suggests a rack-based interface for dealing with effects and synthesizers. Software like Propellerhead's Reason and Arturia's line of soft synths have already used this approach to great effect: individual software synthesizers, meters, and effects can be represented as actual, patchable devices.
An application would represent that rack space, and users would be able to instantiate rack devices (reverb, compression, etc.) to place in the rack, and then be able to patch using virtual cables. The benefit of this is that there is a consistent yet differentiated interface for dealing with effects/synths/meters, and that they have a more substantial presence for the user. The motif of using the rack metaphor would be a convention set forth by the plugin protocol.
I know Jack Rack has already started this idea, but a more graphical approach would, I think, be a great attention-grabber for people interested but non-committal on Linux as an audio platform: many enjoy the substantial feel of software like Reason. The workflow of having one machine running Ardour and another running a virtual graphical rack, with graphical cables connecting the devices over a network is a very exciting prospect, already available from Jack.