OK, but I'm still struggling to understand why you should say there have been very few developments in the last 20 years, but there will be loads in the next 20. I'd have said that there's been slow, but consistent technological progress and it's likely to continue at pretty much the same rate.
Speed hasn't increased, we're still up against the sound barrier. M0.78 is about the right speed for the range of the current narrowbody fleet as over typical distances flown, bumping up to say M0.83 isn't going to make a huge difference on a 2 hour sector where as it would on a 14 hour leg.
180 seems to be about the right passenger load, some variants offer more and some less. Interior capacity for hand luggage needs to be increased as people are tending to carry on rather than pay to check a bag in. The A320 is ahead in this area at the moment but there is still room for improvement.
A new generation flight control system and EFIS would be a welcome addition on the A320 as it currently lags behind the latest generation of flight decks. It's adequate rather than class leading, but as it was designed fly by wire in the first place, improvements should be easy to incorporate. Bringing a B737 up to modern spec would be like trying to incorporate the last 50 years of advances in motoring technology into a 1960s VW Beetle body. As a previous poster stated, things plateaued since the A320 was introduced. Aviation went from the Wright brothers first flight to Concorde in less than 70 years but exponential improvements seem to have died off a bit.
There have been advances in the last 20 years particularly in engine technology (which the A320neo enjoys), composite materials and flight control systems. Due to the expense of aircraft design, a product cycle is much longer than in the automotive industry as manufacturers need to recoup development costs and can't afford to introduce new types every 5 years. Add up the improvements over the past 20 years with the likely advances over the next 20 years and a significantly improved clean sheet design could be offered around 2040 which would probably be all composite material with unducted fan engines and the latest avionics which would be essential given the growth in air traffic density.
Airbus can keep the A320 going until then with a few tweaks here and there where as Boeing needed a B737 replacement a long time ago.