It's one thing to think about a feature but it's another thing to actually test it. That's why it never hurts to make a proof of concept for features you'd like your project to have. That being said, it's best to not spend too much time on them and to only make them for key features. The point of a proof of concept is to test if it works well so you can save you time later down the road.

No first design is perfect, that's why you make a bunch of them and pick the best one. The best tool for this is still good old fashioned pencil and paper. This way you don't grow attached to any design you make and you can still easily showcase your designs to other people for feedback.

To correctly gauge what standards an application must abide by you first have to do research in your target audience. One of the best ways of doing this is directly addressing the source. A well thought out interview can supply you with new perspectives and solutions to problems, moreover it allows you to validate any of your previous work on the project.