The way things are looking for Boeing at the moment, it appears they are having problems with marketing the 757, both the -200 and -300 versions.
Tough competition in the marketplace with it's A321 has been hurting Boeing's sales and even it's own 737-900 may take business from airlines which may have bought more 757s.
With the 777, 767-400 and 737NG having a very similar flight deck is it not time for Boeing to introcuce this onto the 757 and 767-300 range. It would provoide better commonality for the future and the flightdeck would stll be compatible with the old one if airlines wanted that. This would be achieved in the same way as was done to make the 737NG compatible with the "classics," by making the CRT displays capable of displaying the older instruments.
In the cabin the 757-300/737NG interior could be applied to the 757-200 (This may already have been done - it's not clear from boeing.com) More urgently, the 767-300 should get the 777 stlye cabin of the -400, which is considerably more passenger friendly. The 767 cabin design has dated badly, while that of the 757 is still contemporary and modern.
While the 757 may be a very efficient airliner, iprovements could be made here. A couple of years back Rolls Royce proposed an updated RB211 engine for the 757, featuring many Trent components taken from the engine for the new A340-500/600. This would make the 757 quieter, more efficent and improve maintainability and reliability. I'm sure P&W would come up with a similar update to the PW2040.
I'd don't really think the 757NG would need any "metal" changes. The wings and tail are well suited to the 757's task, unlike those on the 737"classic" which wouldn't allow it to fly at higher levels and resticted it's speed and efficiency. However, maybe we could see raked wingtips a la 767-400 to improve long range economy and stretch the 757's endurance.
Boeing could possibly also add extra fuel tanks as is being considered for the 757-200ERX, to stretch it's range a bit further, I'm sure airlines like TWA and Continental would appreciate this.
These changes would help improve the commonality of the Boeing family, increasing the "family feel" of the 737NG, 757NG, 767NG and 777. Boeing has to make up lost ground to Airbus here, they have lead the way recently on Boeing's idea that started with the 757/767 20 years ago!
With regard to Fly-by-Wire, which some have said the 757/767 lack, I don't think they're needed. These aircraft are both exceptionally safe and effivient, there is little to be gained but the cost would be huge.
This brings me to my final point, this would ba a fairly low cost upgrade for Boeing. All the developements I have mentioned are already on other aircraft in the Boeing lineup, so it's saving on the cost of developing from new. Remember, the 727 only had around a 20 year production span, I wouldn't like to see the 757 go out in the same length of time. However, the orders aren't there and Boeing appears to be burying it's head in the sand.
So lets see if you agree/disagree.
Are there any other improvements you can think of?
(I know I've concentrated on the 757 here, but I don't know nearly as much about the 767 and that alrady has a quasi-NG version in the 767-400)