I just came across this tool, which - after entering a city - shows all other cities to which there is a direct train connection. A color code depicts the travel hours.
Although I'm not quite sure yet on what occasion I can use the tool. But someday the moment will come :D
Btw. there is a Sparpreis search on https://bahn.guru/ with a monthly view ("On which day do I travel cheapest from A to B?"). Unfortunately the links to the store don't work yet.