Tool to create a Decision Tree and auto generate Path Combinations

I need to help a team to test a really complex web application. With a lot of big processes. They only have some basic happy flow tests and almost no documentation. They also have no idea how much of the application is currently covered by these happy flow tests. As a start, I think it’s a good idea to create a decision tree. Just so we have a first step towards a test basis, and we know how much of the application is currently covered by tests and which tests we still need to write. I want to follow this approach: https://www.tmap.net/wiki/paths. Off course, we could use a tool like Lucidchart of Mindmeister. But maintaining the decision trees in these tools is going to be a real pain in the ass. Since the processes are so complex and big. The tools also can’t auto generate path combinations and test cases from the decision tree.

So basically I’m looking for a tool that:

1. Can create a decision tree easily
2. Makes a complex decision tree easy to maintain
3. Can auto generate path combinations from the decision tree
4. Can auto generate test cases from the path combinations

Does anyone know if such a tool exists? (Please feel free to suggest different approaches if you think this isn’t the best approach for this situation.)

Thanks! :)