In my personal opinion, I don't believe there is one, and I'm honestly ok with it. There are great many things I enjoy in this life, and I don't need any single unified purpose. In fact having such "purpose" would probably be quite a negative deal for me. What if I don't like it? Not everything needs meaning, some things just happen, I think.
In my experience, people who believe as you seem to, and I've known very many, including a relation who has written prolifically on the subject of Humanism, knowledge etc, as a world renowned intellectual and Doctor , tend to reflect more and adjust their perspective as they approach the end of the winter of life, if they get there, and to develop that caring you don't have on the subject of purpose, future life or no life and God or no God.
What if you don't like it? Well if there is a chance of not liking it, then I guess one aught to ensure that preparations are made to make it as likable as possible, since the duration is said to be infinite.
Yes, things just happen, and a good purpose is to be ever more prepared to accommodate whatever happens with the least displeasure.