The issue is that programming languages don’t go forward, they move sideways or diagonally, or sometimes backwards.
A new car comes out, and it has some cool feature: Hey, it has road surface detection and changes the steering accordingly! But it should also come with all the old stuff that you come to expect. Comfy seats, seatbelts, airconditioner, heated windows, wipers, proximity detection, power steering, cruise control, etc.
With new programming languages, what you tend to get is a chassis, engine and steering wheel, and the road surface detection.
Here is a list of cool ideas that have been discovered and implemented in programming languages, but which do not in their whole make up any existing language:
The moral is, if you’re inventing a new general purpose programming language and you have some clue that it’s going to be adopted, I implore you to thoroughly explore all of the features above within the existing languages that do them well, and think about adding it to your language.
As a follow-up post I might make a matrix of the top, say, 30, general purpose programming languages and all the features that they tick off.