Hi Dr. Stüvel,
my name is Damiano, I’m a software engineer and amateur 3D modeller, and of course I love Blender
I’m working on adding a new rotation mode to objects and bones (and 3D cursor as well): swing-twist, with twist axis being any of X, Y and Z.
I think I have already done most of the work on the core functionalities, and I will work on improving the Python API side and how the rotation mode is visualized in the GUI.
But before doing that, I would like to know if the developers are interested in having this new rotation mode. I think that swing-twist would be more useful than euler, quaternions or axis-angle to use for character animation, because it has 3 components similarly to euler, but it is easier and more intuitive to use than axis-angle and quaternions.
Here (Fundamental flaws in Blender's constraint philosophy? - #5 by Hadriscus), Brecht says that it would be useful to have the swing-twist rotation mode, which is already available for drivers/constraints, in objects/bones.
If you think that this contribution would be accepted, I will keep working on it.
Keep up the great work on Blender!