“Please don't announce projects before you release them.”

I think this extends to those topics where people ask others to work on a project that is not even materialized yet. Sure, collaboration is nice, but without some context from some discussion, people wouldn't get what you're trying to do, and thus would be hesitant to join. Even construction workers need to reach at least some consensus on managing who's going to build what.

Besides, this is a forum, not a business. Every collaborators you can find here are volunteers; they don't gain anything (or anything important) working at your project. (And if you think that you can give them that something I called important, if such are even possible, then perhaps that's a sign you have outgrown Scratch and I can only recommend you to go to a proper establishment.)

If your project in mind requires at least some amount of people to collaborate, then it's not worth doing. That's not to say you shouldn't ask others to work on your project—any help you get is a contribution to your project—but if that is the only source of progress on your project, expect your project to go nowhere. Remember that others have their own priorities in life too.