A free National Rail interface that let me recreate their journey planner. It’s not that their journey planner isn’t good, it’d just be cool to integrate it’s services into my own services.
Websites for events could have a feature that tells customers what the best train (specifically) would be to get to the event. Not just “Get off at Kings Cross station”, but “Get the train that arrives at 13:23 at Kings Cross”. The API could even let the provider book and order train tickets for the customer.
My calendar could automatically update times for services changes or cancellations.
I probably won’t be able to do this one myself. But it’s a cool idea.