A brand new design today could offer some small improvements over the A320 but it's doubtful that there would be the massive leap ahead which would be needed to justify the massive cost of a brand new aircraft and the necessary price increment over the A320. The Airbus can use the latest engines and with sharklets has a very efficient wing, the fuselage is the right size and with additional centre tanks range can be extended.

A few tweaks here and there and the A320 can easily go another 20 years, by then technology will have improved to the extent that it will be worth incorporating the advances in an all new design which will probably be all composite and use half the fuel of the present aircraft. The B737 should have been scrapped in the 1980s and replaced with a design using B767 level technology. Had this been done Boeing would have had an equal competitor to the A320 in the narrow body market .

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.

The economics of a 737 replacement are a different story, I think you're right about that. Even without the aircraft's current problems, Boeing are between a rock and a hard place. If they try to squeeze even another decade out of the 737, Airbus will eat their lunch.

It's going to be interesting to see what type of old school engineering goes into the Chinese Comac 919. It will use the same LEAP engines, but without having to dent the bottom of the cowling to get them to fit.
Might be worth buying one of their aircraft, to study what their design department has come up with.

https://www.youtube.com/watch?v=rfGy9fnqAVo
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There never was a 757-100. Shrinking an existing aircraft and still having it economical is close to impossible. The wing is going to be way too big, manufacturing costs will only be slightly less than the baseline, and seat mile costs will be higher due to too much wing and structure. The 737-500 was a slow seller, the similar sized 737-600NG even worse, the A319 remains a slow seller and the A318 was a major flop. The 747SP was moderately successful, but that was a different time and no other aircraft offered that kind of range (and as the saying went, all they did on the 747SP was remove all the empty seats).
What really killed the 757 was it was expensive to build, as compared to the 737. When the 737-900 came out, it's capabilities were very close to the 757-200 (aside from range) and it cost a lot less to buy. Boeing tried to save the 757 with the stretched -300, but it flopped, in large part because when you make a single aisle that long it takes so long to load and unload that your turn times go to hell and it remained expensive to build and hence expensive to buy.

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.

Read an article a while back that Airbus was looking to develop the wing of the A321LR, presumably to reduce fuel consumption, increase range and bump up the speed. I would expect that if they can achieve this then the A320 family as a whole could see the benefit down the line.

I thought someone said a good while ago this was a field length /field performance issue... Vs provincial airports?
The 737 Vs 757 debate that is. Wing size /wing loading

Undercarriage
 

Boeing delayed replacing the B737 with a new type as they knew if they asked their big 737 customers to transition to a new type, then those same customer would rightfully take the opportunity to conduct an open market competition as to which aircraft to choose, which would include the Airbus A320/A321. Boeing really didn't want their biggest and most loyal customers to start looking around for the best plane.

Most large airlines have a mixed Boeing/Airbus fleet, so 787 and A350, 737 and A321. This way both manufacturers offer their best prices to keep the business and hopefully steal some from the competitor.

G