Long term, Boeing plans on having three families of aircraft, known as Yellowstones 1, 2 and 3. The first Yellowstone family, #2, is the 787. Yellowstone #1 will replace the 737NG family and will almost certainly offer a model capable of doing what the 757 does. Yellowstone #3 will replace the 777 family and the 747-400 passenger model.
However, with Airbus launching the A350 and Boeing launching the 747-8I, Boeing may extend the 787 family greater then planned to replace the 777 family, keep the 747-8I, and push back development of Yellowstone 3.
Through 2015, at least, Boeing will offer:
787 Family, including new members like the 787-10 and 787-8LR
Between 2015 and 2020, Boeing will offer:
Yellowstone 1 to replace the 737
787 Family, including possible new members like the 787-9LR, 787-10ER
Beyond 2020, Boeing will offer.
Yellowstone 3 (possibly not until the mid to late 2020s, if then).