I've been really impressed with the insight in Matthew Dunstan's series of videos on game design.
Obviously he really knows his stuff as he's a published game author (including one of my favorites, Guild of Merchant Explorers) but he can also articulate his thoughts into advice that you can actually do something about. Right at the beginning of the above video he talks about games that start from a good mechanic but then struggle to turn into a compelling game, and recommends melding a theme early on in the process. This is exactly how Neapolitan came to life - it was meandering along as a cool mechanic for a while, but the addition of the ice cream shop theme pushed it into focus. It helped make the scoring and win conditions work, which meant the goals were understandable, which reduced the cognitive load on player turns.

One thing I love about mechanic + theme synergy is that it's one of those spaces where combinatorial glue innovation can result in something quite unique, and the idea space is incredible. When the mechanic + theme mesh well it feels like you're not inventing something, you're merely the person with the privilige of discovering it.