Why do they need a new name? If it's not broken dont fix it
I would go for 737-10, 737-11 etc. which provides a good combination of consistency with the other products but also a sense of development.
While they are about it they might as well switch to 777-2 and 777-3
I thought Boeing had decided to go with 737-7 (737-700), 737-8 (737-800) and 737-9 (737-900) to match the new numbering system of the 787 and 747. That would make more sense than RE737. I believe we will see the 737-7 etc. rather than the RE737.