I think that Boeing's plan will be to replace the 737NG with a new narrow body airliner in the 140-200 seat category that will incorporate a lot technology and advances from the 7e7 models. This new airliner will bring an end to three generations of 737 airliners and eventually replace the 73G/738/739 plus be a true replacement for the 752 (even the smallest member of the 7e7 family is a bit large to directly replace the 752).....I do not expect that a 110 seat 732/735/736 size variant will be a part of the new family of aircraft, that market is being covered by super-sized regional jets like the E190 and CRJ900 and the 736 has not sold well for many reasons.
My guess (and its only a guess) is that we will see a 140 seat version of the 737 successor (replacing the 73G and adding a few seats), a 160 seat version (replacing the 738 which is becoming the star of the 737NG family) and a larger version to cover the 739/752 market. I also would not be surprised if the new aircraft is offered in a variety of range options.....everything from about 2000 miles for short haul up to about 5000 miles for the larger versions to cover thin long range routes. When launched, this aircraft will have to cover a huge market (far larger than the variety of missions to be covered by the 7e7, for example.)
Until then, we may see Boeing try once again with a longer range variant of the 739.....something to keep airlines happy and cover the gap that now exists in Boeing's line-up with the 757 going out of production. A while ago, it seemed that Alaska and some of the European charter carriers were interested in the 737-900ER, but there has been very little news on this variant in recent months and Alaska recently swapped some 739 orders for the 738 (mainly to fly SEA
-Florida routes where the 739 has limitations) so it is very questionable if the -900ER variant will ever be launched.