It will be cool to see that in Nomad - This feature is avaliable in 3d coat and Quick shape for Blender by Jama.
The main Idea is you can choice some form (It can be Rectangle/ Oval/ Or maybe Custom Alpha brush) and just draw in 3d space or on 3d Surface!
Check this video to see how it works in 3Dcoat:
In Nomad Drag tool with high intensity does that.You can use stamps too.
This quick curve is quite close to the curve we already have isn’t it?
There could be add one to tube like:
-Pressure to radius
-Autovoxel with objects named tube / with Sub and Add mode.
-create surface option for closed tube, instead of a tube it generates a extrusion of drawn shape, radius becomes extrusion height. Together with post subdivision it would be mesh balloon style.
Easy written down ideas, probably a nightmare to program.