If DL were to start a SIN route that does not go through NRT, the most likely gateway would be LAX, followed distantly by SEA.

DL's currently longest flight is ATL-JNB which is around 7300nm and it is routinely weight-restricted. JFK/DTW/MSP-SIN would all be considerably longer than that and, while within the operational range of the 777-200LR, it would not be able to regularly carry a full compliment of passengers & cargo. ATL-SIN is completely out of the question.

DL is building up LAX to be a major hub for business travel and as such, it would make sense for them to offer a better timed one-stop flight to SIN via LAX. Most of the major business markets in the US now, or will soon have, nonstop service on DL to LAX making for a relatively painless connection.

SEA is shorter than LAX-SIN, but has less connection opportunities for many of the major business markets in the U.S. It's a possibility, but a distant second to LAX in my opinion.