Sorry to disappoint you, but before MI 1 production ended, I had planned it as a trilogy. Don’t mistake “I didn’t have an ending” with not knowing what the theme of the ending should be and where the overall story needed to go. It’s how it works. Some people will tell you you have to have the ending before you begin, these people are just wrong and aren’t people who create (interesting) stuff. Part of the process of writing is understanding your story. By the time you finish, you realize it’s not the story you started with.
If you want to think that I had the whole MI trilogy in my head from day one, you will be disappointed. I knew large story beats, but that’s it and that’s all I would expect most story tellers to have. Ideas are guides, they are not a hard map to follow. It’s a process called creating. Good idea really come out fully formed.