The US designed a number of prototype heavy tanks, in addition to the experimental T20 medium series that eventually led to the Pershing. Due primarily to shipping constraints, the US never got heavies into mass production, though they did eventually get the Pershing, which was arguably a heavy though it came from a medium tank lineage, out there in comparatively small quantities.
The challenge is to get something heavier than a Pershing into series production in time for it to be in the hands of troops by the time of the Battle of the Bulge. It doesn't have to replace the Sherman. There just have to be some present at BOB. Some possibilities: The M6 (probably up-gunned to 90mm gun), or maybe an earlier development of the T29/US T34 (very formidable tanks that barely missed World War II).
So, why does the US go with mass production of a heavy tank? Which one do they go with? How does having a few hundred or a thousand of them impact BOB and subsequent battles?