Suggestion Box

With the exception of a few "Official Writing.Com Contests," most often "Short Shots: Official WDC Contest"  [ASR] and "Quotation Inspiration: Official Contest"  [ASR], contests on Writing.Com are not run by site staff, so it'll be up to a member to create a contest. While I don't believe I've seen an actual contest of the sort you're describing, some moderators of Interactives state in their introductions that they'll reward what they feel to be quality additions, and may even award Merit Badges once a person contributes a certain number of additions. You could do this with your own Interactive, or, if there's someone else's Interactive that you like but to which you don't want to be the only one adding, you could say that you'll reward people with gift points and/or Merit Badges for adding chapters to that Interactive.

In any case, if you want people to be rewarded for adding quality chapters to Interactives in an effort to get people posting in them, your best bet will be to take the initiative.