Strange Awakening: Initial concept was: "What if a man woke up in another world where he'd been born female." It kind of morphed into something else quickly.
The Jack Diamond stories: Overall: "Can a 'classic noir' story feature a superhero as the detective?" Book one is: "How does this detective deal with discovering magic is real, and gaining magical items?" Book two is: "How does the detective deal with meeting vampires and other superbeings who are not demons?" Book three, if I ever get to it, is "How does a dark and gritty detective (with powers) deal with a traditional (though flirtatious) 4-color hero (and a demonic serial killer)?" four would be "How does the detectives support cast deal with him being taken out of commission, on death's door, thanks to a demon attack?" and there would still be at least one more in the series.
True Blue: Overall: "After his death in an alien invasion, the memoirs of a superhero who spent most of his life posing as a detective fall into the hands of his child, his Legacy." Each individual story would have a different thesis. There would probably have been ten if I'd been able to keep up with it but...
The Kaiju System: (On Honeyfeed - though I did make some joking comments about it here before writing it there) Initial concept: What would be the craziest, silliest thing I could do with a System. Eventual thesis: "What if a young man suddenly stumbled on an ancient secret that could potentially turn the tide in an interstellar war?"
Digital Cowboy (On PocketFM as Digital Cowboy Dane): "What if someone were isekai'd into not a traditional medieval or generic Asian feudal era fantasy but a Western?"
Between Worlds (On PocketFM as Between Earth and Pyrroth; full title should be: "Between Worlds, Book 1: The Lost Arch Mage"): The series thesis would be: "What if an ancient prophecy, spanning a sixty-year sequence of events, brought people from the modern world to team up with people from a traditional fantasy world to save all of Creation?" Book One's thesis is: "What if a group of gamers discovered the game they played is based on another, very real world - one that expected them?"