I think they could work by making just a button that activates 3d grips like the 2d that already exists, even better if you right clic one of those 2d grips the 3d ones could appear!

Recently I've been many times in the same situation: I have 2 or more curves and I want to draw something (a polyline, a circle or an interpolated curve) that touch those curves but with all the points sharing a single plane. What i have done is to copy those curves, trim them with a plane and then used the end points of that new trimmed curves for snapping the thing I want to draw. Those are a lot of steps for something as simple as drawing a circle.

What do you think about this feature?: Making two construction lines that starts on the same point makes a temporary cplane. I think this could be useful in many situations not just the one I described above.