I don't think the replacement for the 737NG will be called a 737. I don't even think it will be just one plane. The Next Generation 737's are very different than the originals. The original 737-100 and 200 series were intended for short trips often to aiports with little ground equipment. The Next generation 737's were designed to fly coast to coast, replace 727's, and even fly ETOPS 180 routes. Boeing needs to have a light 100 seat plane that can compete against Embraer and have a 200 seat plane that can replace the 757-200 on long thin transatlantic routes.
I don't think it will be called a 737 either but right now, for lack of a better name, some are calling it the 737-1000.
Slightly off topic, but is the 737 design also being considered for an MPA (maritime patrol, P-3 type mission)?