Well, I added "The Lou" for two reasons - 1) Some other person had started up a /c/saintlouis community (and let it dwindle) and I wanted this one to visibly stand out from that one in search results, and 2) I also started /c/missouri and named it "Missouri | The Show-Me State", so I wanted a nick name to go along with this one as well. I haven't heard anyone call it "St. Louie" in a hot minute, so I went with a newer one.

https://www.quora.com/What-is-the-city-of-St-Louiss-nickname-and-why-was-it-given-this-name

What do you think of using "Gateway to the West"? That's probably the most tried and true city nickname I know of.