I have a soft spot for reindeer, having previously written "Was Santa Claus the First to Domesticate Reindeer?" and "The Reindeer Superpower: Ultraviolet Vision".
As always, I also publicize my article in DeHaan Services, although "Reindeer Keep a Cool Head" does not add much to the discussion.
Keep an eye out for new variations on your previous themes. In this example, I had written about reindeer a couple times before. When I saw the news item about the reindeer cooling system, I knew that I already had some images for the topic and some background knowledge.
This differs from "plan and write a series", because you don't have to start with a group of articles in mind. Just keep an interest in old topics and see whether you can say something fresh about it.