Y1: 737-752 replacement
Obviously a single aisle narrow body to replace the 737, i'm thinking a smaller 787 all around. Same technology but put in a smaller package. What is uncertain is how low of capacity the plane will be made to replace. The 736 wasn't really the best seller, more airlines are going for planes like the E190, so will boeing try to compete with that or build an entirely new plane to compete with that plane.
The 752 replacement could be a twin aisle version, something like the 762, a short wide plane but it would probably be more like a longer version of the 737 replacement.
I'm thinking this will be named the 797. So the 737-700 will be the 797-1, the 737-800 is the 797-2 but the 737-900 and 757-200 will become one plane, especially if they share the same cabin width.
The Y3 might come earlier though with the way the A350 is looking. The A350 is looking like a serious competitor to the 777 and Boeing does not want to let that segment go. The 777 is a great airplane but the technology is getting a little old. To compete with the A350 they have to offer at least the same advantages and make it a little better. They may go with the full Y3 or maybe just a big update to the 777 and make the 777NG, just like they did with the 737NG. The technology might not be there to really make a complete Y3 that could cannibalize the 748. For the 777NG they would use composite frames, maybe barrell technology but composite panels would probably be easier and cheaper right now. Use GenEx engines, GE
will have to get over the fact that the GE90 might not have been fully paid off but they will have to deal with it, possible same engines as the 787 will give engine commanality, and similar cockpits will make pilot training easier for airlines that order both. This might be expensive but it will be cheaper than creating an entirely new plane family (the Y3) and a stop gap solution for the A350 until the Y3 can be completed. The 777-400 will replace the 772 and a possible LR
version, but with the saved weight by using composites and new technology the range would already be great with the 774 that a 774LR would be able to fly extremely far, then the 777-500 would replace the 777-300, all roughly the same capacity.
A 787-10 could happen but stretching the 787 to a -10 and -11 is not that smart, the plane is meant for the -8 length and stretching it that much could turn out like the A340-600. the A346 is a good plane but that huge stretch gave it some balance problems that impacts cargo volume the plane is able to carry. Now if the entire 787 was upscaled (Fuselage width increased and stretched) then the proportions would work out better, but then you might as well call that the 777NG