Sometimes you don't need a collaborator if you have the idea. If the other party is not at all working on the angle that you're interested in, it's probably not the correct collaboration to ask to.

Also, a collaborator is usually not a stranger over the internet, it's often someone who you know and you already worked with, so it is not that ackward to expose a new idea and propose to work together.

It takes time and social skills to make long lasting collaborations, the two parties must trust each other in order to collaborate. In this context, exchanging ideas is not really an issue.