I'm surprised that nobody ever came up with the plans to buy surplus 727-200s and re-engine them with the Tay or something similar, then market them in countries which might not be able to fund newer types.
UPS re-engined their fleet with Tays... whether sucessful or not, you'd have to ask someone else.
Now that the last 757s are coming off the line which aircraft duplicate its versatility and performance? The B739(X) and A321 don't seem to cut the mustard. Perhaps a version of the 7e7?
Boeing will likely convince a good number of 757 carriers to go for the 7E7-3. Reason being, many bulk 757 opperators are also bulk 767 opperators (think DL
, ect) so as they will be ordering 7E7-8s, a 7E7-3 order would fit. The 7E7-3 is a decent jump in capacity over most 757 configurations so Boeing must provide some product beneath the 752 and above the 738/739 to fill this gap.
Short term, Boeing might start the 737-900X as a side project. This would provide many 757 carriers an ability to add-on to their existing fleets as new 757s are not avialable. Since Boeing is going to have to re-engineer the 737 platform to higher weights for the MMA project, now might be a good time to carry that military work over to the commercial sector. When the 737 is ultimatly replaced, Boeing will probably get the 180-200 passenger variant "right" and not end up with another 739 with limited market appeal.