I feel like OC bios are a good starting point. Kind of a getting-to-know-you type thing as people join up and get used to the DW format. But recurring events might help encourage shyer people (myself included) to post and participate - I feel weird just posting things in communal/semi-public areas without prompting. (I've barely ever posted to the SWTOR tags on Tumblr in all the years I've been over there, sooooo.)

Other ideas:
- Something like OC ask memes to get people talking about their characters and interacting with each other ABOUT their characters
- Fic prompts for the writerly types
- LFG for roleplayers