I've been using this work-in-progress web app that uses only lines, fills and shapes to create human characters. It has some bugs and has a different style to Mannequin, but it can create some cool results:
Just wanted to share this as inspiration for options / functionality that could be adopted into Mannequin if anything seems interesting in it.
Heres a link to it: https://perplexedpeach.gitlab.io/dynamic-avatar-drawer/dist/test.html
(info page) https://perplexedpeach.gitlab.io/dynamic-avatar-drawer/