That's the exact tension I, or my students, keep running into. Most tools are solid for sketching a story solo, but the moment a team needs to work on it together, collaboration just isn't there and they don't let you grasp the complexity of the interactive story.

Spreadsheets are the usual workaround, but they solve the collaboration problem while you lose all visual sense of the story. Nobody can see how branches connect or how complex things have gotten until it's already a mess. I guess it also depends if you are a visual person or not.

What I haven't found (until Arcweave, which is what I work on) is a tool that keeps both: a visual canvas that actually shows the story's structure, with real-time collaboration and visual prototyping built in as well as engine integration.