You might take a look at the top stories to see if any patterns show up. I think that broad character and setting appeal can help if you're looking for more views and collaborators; I'd give
@duskford 's
At The Cabin high marks for both views and contributions from multiple authors.
A compelling story + character can help. Take a look at
@hematoma 's
A Virgin Heroine for another winner in views and contributions.
Another model that has seemed to work well in getting contributions (though more in long strings from one or another author, rather than collaboration among authors) is
@SwampThing 's
Slut World, which overlays a set of rules based on the Apocalypse World tabletop roleplaying game, translated for telling sexual encounter stories. My story
Lusty Magical Academy is a direct take-off of Slut World with rules tailored for a Harry Potter-esque setting (though for a college age range).
A rich, deep, and stylized approach gets more readers than it does contributors. I think my story Lucy's Portion leans this way, as do (even more so)
@Beeble42's
richly literary stories (click "Stories" on his Chyoa user tab) Decameron XXX and Cockshoots Hall and the sexed-up BBC dramas in Sex Party Britain. I love them and also find it hard to know where I might be able to contribute, other than cheering the author on and offering a suggestion here and there.
And you will typically have many views, few votes, scarce comments, and rare contributions. This is a little hard on me since comments and contributions are the best reward. Again,
@duskford has been a supportive editor who frequently comments and adds suggestions, so I have gone back to At The Cabin again and again.
I seem to be better at getting views (partly through frequent updates) than I have been in getting collaborators, at least in recent times. So if you learn something that works for you in getting more contributed threads, let us all know.
Maybe someone else could guide you in your questions about optimal structure and tone. As someone who has been a contributor to Chyoo for, um, over ten years, I can only point to examples and repeat what other authors have observed.
You're free to write to please yourself, and so you may want to lean toward what keeps you interested in contributing. Or if you get more out of collaboration, you might want to find stories you like reading and branch off decision points with scenes you'd like to see. I've especially enjoyed the times when I've been able to alternate pages with another writer to build up a storyline. Or you can find a story idea that might bring in other contributors, maybe a fantasy adventure or a sexed-up slice of life romp.
So: you may have to answer your questions yourself, but that's my perspective. Maybe some other folks have suggestions to offer.
-Z.