Keesje, Derg:
According to this article by John Ostrower, Mike Bair has stated already that the 737 successor will have no open rotor but rather a conventional-looking engine:
http://www.flightglobal.com/blogs/flightblogger/2011/02/clean-sheet---boeings-all-new.html#more
That should limit the potential fuel burn reduction to a few per cent beyond what the LeapX and GTF can do - essentially the next incremental upgrade. Also, the conventional tube-and-wing will probably remain. That said, if you look at the projected empty weight of planes like the Cseries, it's clear that that'll pose a challenge to heavier aircraft like the current 737/320, at least for the lower end of the market. Then OTOH, Boeing will also at one point want to replace the 757 and even 767-200 with their new model, so it seems to me they might go for a 7-abreast design and try to make that light. It would place them in a bracket just above the new entrants.
I'v also just read in Aviation Week that at least one large 737 customer is looking at cheaper, smaller planes, so that may put my above reasoning into doubt:
www.aviationweek.com/aw/generic/story.jsp?id=news/avd/2011/03/01/01.xml&headline=Ryanair%20Considering%20C919s,%20MS-21s%20For%20Fleet&channel=comm
Concerning Airbus, at my last count they had 202 commitments for NEO (counting MOU's) and have a long-running effort under the moniker "A30X" for an eventual clean-sheet 320 replacement, so it seems they are taking a two-step approach. I wouldn't be surprised at all if it all ended in a continuing up-and-down between the two, first NEO, then 797, then A30X in maybe 2028, and so on.