Modo has gotten the ability to edit the motion path directly from the viewport. Cinema 4D and Maya has this as well.
Is there anything like that in Blender? What would it take to build it?
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There is an add-on for that called Motion Trail that allows you to edit the curves in 3D space. You can enable in the User Preferences under the Add-on tab.
There is a reason that there isn't a real-time motion trail editor addon, the current desp graph makes a mess of the data, especially with IK factored in. It seems that it is computationally impossible the way that the animation data is stored and used with Blender. They are working on a new desp graph that I think will allow it, but don't quote me on that.