Some research from Microsoft showing interesting ideas for new multi-touch object manipulation gestures.
“Direct touch manipulations enable the user to interact with the on-screen content in a direct and easy manner closely mimicking the spatial manipulations in the physical world. However, they also suffer from well-known issues of precision, occlusion and an inability to isolate different degrees of freedom in spatial manipulations. We present a set of interactions, called Rock & Rails, that augment existing direct touch manipulations with shape-based gestures, thus providing on-demand gain control, occlusion avoidance, and separation of constraints in 2D manipulation tasks.”
Link: Rock & Rails: Extending Multi-touch Interactions with Shape Gestures (microsoft.com, via)
One issue I see with these gestures is that in many cases they require whole-hand gestures and a large workface to match. These qualities are excellent for Surface-scale devices, but would appear to be impossible to scale down to portable devices.
I urge you to expand your exploration to cover similar gestures that involve either finger-joint or clustered fingertip gestures.