Backstory: I've used headwayapp.co on multiple projects in the past, it's a public change log for customers. Recently I had friend inquire about a tool for future project planning similar to headway, simple and inexpensive. I couldn't find anything from a quick search.
The Idea: A simple public page for SaaS companies to display their roadmap.
Deep Dive: Similar to headway, you would build a simple interface where users can add dates and features to predict releases. Since software can vary, you should add a date range or rough date approximation.
Validation: There are a lot of internal team roadmap pieces of software available.
Monetization: You would charge a small monthly fee for this service.