So i got 2 additional ideas that might be relatively simple but pretty powerful.
2 new chisel modes that i think could be an awesome addition.

1. Circle
   No, i do not mean SPHERE. Its based off the "plane" mode, but its a circle, where you can select the radius. Maybe even set it Manually by scrolling with the mousewheel or a certain key combo? Max. radius would be 8 of course, and min maybe 3?
   I'd not make it so the circle is fixed in the middle, but placeable freely with the selected radius. This would make creating round parts very nice and handy without a lot of struggle. Especially the setting of radius could be very awesome.

2. Square
   No, not plane. But kind of. My idea here is a SQUARE. Freely placeable/chiselable, maybe also with the ability to set the lenghts? (a/b) Or maybe just fixed sizes selectable. The key difference, is that you can chip away or place smaller squares within blocks, but with just the thickness of the plane mode.

So you could say, both of these are an addon to the plane mode. I think those two could be VERY nice and handy. What do you think? Summary:

1. Circle
   Plane mode/2D Circle, Radius either selectable or determined by user input somehow.
2. Square
   Pretty much the same thing, but as a square.

Please lemme know if you dont fully get what i mean. ^^

Add:
I'd male them own categories. Just BASED off plane.