immunity for all potential active and future investigations that uncover more crimes

If they were doing things by the book this wouldn't be an issue. Typically a plea deal involves the defendant fessing up to all the crimes they committed. Only then is a deal cut that decides which crimes to charge and the potential sentence. That's where Michael Cohen ran into issues. He was all too happy to fess up to his crimes when he thought could be used to go after Trump, but he wanted to keep the rest of his sleazy dealings under wraps.

immunity for all potential active and future investigations that uncover more crimes

If they were doing things by the book this wouldn't be an issue. Typically a plea deal involves the defendant fessing up to all the crimes they committed. Only then is a deal cut that decides which crimes to charge and the potential sentence. That's where Michael Cohen ran into issues. He was all too happy to fess up to his crimes when he thought could be used to go after Trump, but he wanted to keep the rest of his sleazy dealings under wrap.