There are a few challenges I've seen on Twitter.
One that I recently participated in was the Draw your friends OCs challenge. That one was pretty fun and really great for drawing in new people. Who wouldn't want a free picture of their character?
The other one I've seen around is you ask someone to send you 3 random emojis and you have to create a character from it. I haven't participated in this one yet but I've seen really unique designs!
I'm sure there are more but those are the only two I can think of off the top of my head!
Hope this helps