Joy of Underconfidence for Founders
What was a simple business idea you had but thought it would be too vague to implement, only to find out years down the line a multi-million dollar business operating on it?
That said, back in about 2008, I was working a rather boring development job and pitched to my friend and colleague, who was a sports fan just like myself, that the best part of fantasy sports was the draft at the beginning and that the rest of the season was rather boring. That I'd rather do a daily draft of players playing that evening. We talked about building a prototype and I think I got as far as writing the statistics scraper. Anyway, it fizzled pretty quickly.
A few years a later, DFS is a huge thing. I didn't have this idea first, not by a long shot (https://en.wikipedia.org/wiki/Daily_fantasy_sports#Early_exa...), but it certainly wasn't a mainstream thought back then.