What if the United States had gone communist at some point in history? Here is my guess at how the flags of the United Socialist Council Republics of America could look like.
1A. Keep the same flag as before. This was common in many Stalinist countries - especially if the new leaders wanted to tap into nationalistic sentiment. Czechoslovakia is a good example.
1B. Strong continuity with the past while still clearly stating the new politic-economic nature of the union. The canton represents the many states of the union working together to achieve the common goal of building socialism in America. Using already existing flags with Stalinist additions was also common in Warsaw Pact member states. East Germany or Romanis are good examples of this.
1C. Same as 1B just with a red star instead of hammer and sickle.
2A. A clear break with the past while maintaining some continuity by using the traditional red white and blue.
2B. Same as 2A but with a design more resembling those of the republics of the USSR.
2C. A more "Chinese" version. The traditional colours has been replaced but the 50 stars still gives some continuity with the past.
3A. A complete break with the past. This simpler, more revolutionary, style is taken from an old flag of the Georgian SSR just with the initials substituted.
3B. Can't be much more simple. A red banner with the initials of the union in the canton. The sort of simple flag loosely organised revolutionaries would stitch together as a practical measure.