A good topic creates a choice
The best impromptu speaking topics make you choose, explain, compare or defend. A flat topic gives you nothing to organize.
Use categories
Rotate between opinion, interview, workplace, funny, explain-like-I-am-five and story prompts so your speaking range grows.
Answer, then retry
Speak once for 60 seconds, read your feedback, then retry the same topic with a cleaner point and one stronger example.