Well, you have to keep in mind that we have a long time before we run out of 7X7 designations. I mean, we are still using 737 after more than thirty years. and 747. And the desig 757/767 is also twenty years old.
I do however think that this is the last generation of 737. I don't think we will ever see another version. I mean, what would it be, 737-1000? That could be confused with 737-100. Okay sure, none of those still exist but none the less, you see the problem.
I for one am hoping for a T-tail design with tail mount PRATT AND WHITNEY (they build the best, but no one uses them) engines designated 787.
Just my twenty bucks.