"Impromptu topic" is a term that could be used to describe a topic that is spoken about spontaneously or on the spot, such as during a speech or presentation. For example, you could say: "I was asked to give a presentation on a topic of my choosing, but at the last minute, the organizer asked me to switch to an impromptu topic that they had just thought of."
It's also possible to describe this situation as a "spontaneous" or "unplanned" topic, or as a topic that is "off the cuff," which means that it is not prepared in advance.
For example: "I had to come up with a speech on an impromptu topic at the last minute, so I had to think on my feet and speak extemporaneously." "I wasn't expecting to speak about this topic, so I had to come up with something spontaneous." "I had to give an off-the-cuff talk on a topic that was not planned in advance."
It's also possible to describe this situation as a "spontaneous" or "unplanned" topic, or as a topic that is "off the cuff," which means that it is not prepared in advance.
For example: "I had to come up with a speech on an impromptu topic at the last minute, so I had to think on my feet and speak extemporaneously." "I wasn't expecting to speak about this topic, so I had to come up with something spontaneous." "I had to give an off-the-cuff talk on a topic that was not planned in advance."