Fidea

Idea sharing on steroids

Features

Share ideas like posts.
Add onto other's ideas: Other people can create branches like github to add onto your exisitng idea, you can choose to merge their idea with your existing idea.
Interest ideas you like: People can interest your ideas. The ideas with most interest will rank in the leaderboard.
Contribution: Developers or other people can contribute to implement your idea.
Progress: A progress bar will show the progress of your idea's implementation.
Contribution Rate: Other people can see who is contributing in your ideas with percentage of their contribution.
Weekly/Monthly challenges: a real world problem will be shared and the users can post their ideas to solve that problem. Best idea gets featured on home page.
Random idea generator on various/selected topic: User can choose a topic and the app will generate ideas on that topic with specified resourcese e.g. the app may tell you to create a landing page with a specified style, font, color pallete etc.

Or use Firebase for complete backend (again, poll to choose what contributors prefer)