Honestly? WIPs.
Notice how you jump onto a project when someone isn't promoting their comic but showing concept art? Ever notice how you run and ask "who is this angel" when you see someone post sketches or even just doodles of characters and then they say "Oh this is X for Y's comic"?
People like Work in progress.
We like seeing people actively working on something, being excited about the process, and letting others in on it. It feels intimate. The number of people who followed my work were excited when I was posting doodles and concept art. I had a conversation about my characters, I even showed snippets of script. And when I finally posted updates, people were like "DANG! You really changed xyz" or "Z totally looked like that type to do this"
I dunno -- I don't think it's all that crazy. I just feel it's really a matter of letting people get a glimpse of your process, seeing how you think or how you do your thing. You give them a reason to care.