How about this? Early in the 20th Century, Bible translators, wishing to fit in with modern scientific knowledge about the age of the earth (Creationism wasn't such a big issue in the US before the 1920s) use the word "era" rather than "day" in the English translation of Genesis (so God creates the world in seven eras, not in seven days). This would actually be a more correct translation (as most of the early Church fathers such as Origen or Augustine knew) since the Hebrew word here translated "day", when used without a definate article means "indefinate period of time" (the same way a phrase like "in King Harold's day" wouldn't refer to a single 24 hour day) and in Genesis it *is* used without the definite article.
Since modern translations of the Bible, including the New International Version used by Evangelical Christians, would no longer appear to claim that the earth was created in a literal seven days of twenty-four hours, evolution might become a non-issue in the US, the way it is everywhere else in the Christian world (or was until recently, with US evangelists being fixated on the issue and wanting to spread the good news that Darwin was wrong to other nations). More likely, creationism would still exist, but it would be "old-earth" creationism rather than "young earth" creationism.
Alternatively, what if some genius comes up with a good solid theory reconciling biblical creation with scientific evolution, and the theory of original sin with our being descended from apes (I know there are LOTS of Christian scientists today with ideas as to how this is possible), and, say, publishes a best selling book on the topic that gets his ideas accepted into mainstream Christian American thought (let's say in addition to being well read in both science and theology he's a brilliant writer with a talent for self-publicity and a modicum of luck).
Alternatively, conservative Christianity could go into a decline. Liberal Christians today are mostly rubbish at converting and inspiring people, but this doesn't have to be the case. Look at George Fox, founder of the Quakers - an extreme liberal (for his time) who inspired thousands to reject the established Church and re-dedicate their lives to Jesus. How about a liberal religious revival sometime in American history?