Giving a talk on a different topic than what we discussed
If you haven't come to an agreement on what you would discuss, then I think you are pretty free. But as a courtesy, if nothing else, send him a note about your topic. If he disagrees you might want to re-think it. 

However, a talk about partially unformed ideas, if presented that way can be very valuable. It can give ideas about research the listeners might explore. If you do it right, it can give ideas about how to come up with such ideas and how to begin to tackle them. What insight do you have into the problems? 

In the best case it can lead to collaborations on that topic and others that might emerge from the exploration. 

In fact, a talk on "things worth exploring" is probably more interesting to a mathematician than one on "things done and finished."