I have all of these guys UFO, and most I'd be totally down with trading (only a couple are USD only)

As for art challenges, I pretty much only have the ones I listed last time..!

1. Monochrome - drawing the OC all in 1 color (such as only blues)
2. Palette challenges - drawing the OC with a specific palette, such as these
3. Actually there's a ton of art challenge things on tumblr like this one (You can also use a random number generator instead!)
4. This pinterest has a ton of challenges, though imo they look more like design-a-character rather than draw-a-character (if that makes sense)
5. There's some outfit ones too, where you draw the OC in a specific outfit. 1, 2, 3, 4
6. Draw the OC on a specific canvas size, like a really small canvas.
7. Then, of course, draw the OC in a limited amount of time challenges
8. & with specific brushes, like binary (pixel) vs, like, crayon

(I didn't check all of the blogs I sent btw..! The links should take you to the relevant places, tho)