I have a contest where you can participate by sharing any episode anywhere. When my comic https://tapas.io/series/Death_and_the_Maiden hits 1K subs, five contestants win a mug from my shop at Redbubble. This costs me more than I'll ever make on Tapas, and doesn't work at all
If I was to do another engagement experiment, I would put out some of my untranslated episodes (they are originally in Norwegian), and ask subscribers to fill in their own dialog. There are a lot of talented and funny people on Tapas, and I have often received alternative jokes in the comments field.
I have also included fellow creators (who are also subscribers) characters in my comic. This was popular. Especially with subscribers of both comics.