The Vision Ends Today
You found a compelling problem to solve. There's a considerable audience that wants it solved. Your solution is decent, and people are using it.
Where do you go from here? Fast forward 4 or 5 years, and think about the problem you're solving today. No, I mean, think again. What's the broader picture? Is the thing you're tackling the real challenge for users? Or is it part of something bigger? If so, are you capable of solving it as well?
A product is a vehicle for creating change in the world. Think about that. Come up with a picture of a future where you fully solved the problem for your users. Describe all the fantastic things that now they can do because your product exists. How were their lives improved? What have they accomplished?
Now work backward to fulfill that vision. What are the logical steps for this wild future? Keep working until you connect it with your next sprint planning. Finish (and unleash) your vision by connecting it to today.