I haven’t read anything about selecting local teams. They built it, fans will come regardless of teams. STL and Cubs is an iconic historical rivalry though (sort of), so they can throwback right into the era and it looks right.
They should do Dodgers / Giants with the Brooklyn and NY Giants jerseys. Nats (Senators throwbacks) vs an AL East team or the Philadelphia Athletics (both were AL in the early 20th century), Indians/Tigers, etc.
Assuming St. Louis will have St. Louis Browns throwbacks? STL vs Detroit would have been a good one, since they were rivals back then.
The possibilities are endless for this.