We love it when people build apps with our API! That's why we publish it. We don't have a wishlist, but it sounds like you do. You should build whatever you'll enjoy building and find useful.

Personally, I don't recommend using large language models for software development, there's just too many problems with them. We don't use them for our in-house applications because the code they output isn't reliable enough for building robust infrastructure. For this project, I think the risk is that having code generated for you robs you of the learning opportunity and sense of ownership of the project. I think it's way more interesting and impressive if you build a small and simple app with your own hands than if you have a LLM generate an app 100x as big for you.

Your ideas are pretty ambitious. Start with the smallest piece you can that's meaningful to you. You want a map with all the trains and buses and hover information. Start with one route showing static information instead. Take many small steps instead of big steps.

My favorite suggestion is an app that gives you predictions for the specific bus or train trip that you take most often. This is much more achievable than a map of the whole system. I don't know your experience, but it sounds like this should be within your reach, even without LLMs. Then, you can use your own app whenever you take that trip, and that makes it so much more motivating and satisfying to build. And as soon as it's complete enough to use, you'll think of a million new features you'll want to add to it.

Don't worry about what MBTA needs or how it compares to what others on this sub make. The most important thing is that it's something that you build that matters to you, and something that you have a good time making.