I've been playing around with @AnTi's new offset code and have done a couple of brushes with it. You'll need my tweaked version from my Github for the brushset to work, see discussion at the end of this issue. Get the brushset here.
The brushes look like this:
(Well, the dry bristle brush just uses the existing code, but it fits the theme.)
Icons are still very preliminary. I'm open to suggestions, tweaks etc. It's possible that you experience some jitter of the brush radius, especially when putting the pen down. The code will probably need to be tweaked to avoid that, but "unfortunately", it doesn't happen for me so it's hard for me to guess the correct values for this. EDIT 12 Feb 2017: I have updated my branch to avoid jittering.
A painting I did that makes heavy use of some of those brushes: