Nah. I think that MLS could get away with "St. Louis Athletic" if it wants because it's a different name and sport from "Philadelphia Athletics," and "Kansas City Athletics" and "Oakland Athletics." I don't think that you could call the team the "St. Louis Browns," however - because that name must be trademarked by either Major League Baseball or the Baltimore Orioles. But, I suggest that it would be a better move for you guys to name your proto-MLS expansion team with an original wordmark like the "St. Louis Voyagers," "St. Louis Archers" or "Saint Louis F.C."
STL FC is already the USL team, so unless they promote it's not gonna happen Archers was a WPSL team; I don't know the status of that trademark, but even if it's open to take I'm not sure reviving a defunct WPSL name would float the boat for an MLS team name Voyagers... eh, mentioned before I don't like that one.
I like the name St. Louis Spirit. Team colors would be yellow and white with blue accents and a secondary uni of red. Obviously a shirt sponsor of some AB product.
Ohok I went a little weird but I looked at really old defunct teams in STL and at teams of sister cities there are my findings: North American Soccer Football League- St. Louis Raiders (46/47) NFL- St. Louis All-Stars (1923) NBA- St. Louis Bombers (46-50) Indep/NFL/AFL- St. Louis Gunners (31-34/34/38-39) ASL- St. Louis Mules/Frogs (1972) MISL- St. Louis Storm (89-92) Samara (Russia): "Krylia Sovetov" means "Wings of the Soviets"- St. Louis Soviets Stuttgart (GER): Stuttgarter Kickers- St. Louis Kickers Saint-Louis, Senegal: St. Louis Senegals Lyon (FRA): Lyon OU Les Loups (The Wolves)- St. Louis Wolves/Loups Galway (IRE): Connacht Rugby The Devil's Own- St. Louis Devils Bologna (ITA): Bologna F.C. 1909 I Veltri (The Greyhounds)- St. Louis Greyhounds San Luis Potosí City (MEX): City of Gardens - St. Louis Gardeners Nanjing (CHN): Jiangsu Sainty- St. Louis Saints/Saintly Saints Nanjing (CHN): Jiangsu Dragons- St. Louis Dragons Brčko (BnH): FK Lokomotiva- St. Louis Locomotive/ Locomotive St. Louis Szczecin (POL): MKS Pogoń Szczecin Portowcy (The Dockers)- St. Louis Dockers Some work, some stink, some are weird and some would be unique in american sport!