Indeed, Boeing has chosen to re-engine the 737. But there're no specs for it yet at all. AA, in a way "ordered the right to order" such plane, but they don't even know what can come out of it.
there are also no specs for the NEO, just a projected decrease in fuel burn by 12-15%. AA still wouldnt commit to an order without any type of gaurantees.
So you are saying that if there is going to be a 737 generation 4, it will have to be a bit wider in order to have the room for what is needed to make it taller (Like landing gear and the new engines themselves, for instance).
The 737 has already been raised a few inches from when the NG was made. The clearance needed to put a full size (76 or 78 inch i believe) Leap X or GTF engine under the wings was 8 inches. That increase was possible with simply moving the nose gear down 8 inches like airbus did with the A330F. It also would have had the blister to house the gear. However, to do that would still have been a lot.
As for tailstrikes, they dont happen very often, but every plane (even airbus') can have them. Thats a pilot issue though