I was briefly a commercial APL programmer, after APL blew my mind in college. (An IBM 1130 running Fortran punched cards was the competition.) How much mileage one can get from "Every problem as an array problem" was APL's singular lesson for me, to this day.
In the same idea box for me: Monadic parsers in Haskell can be mind-blowing, but every problem isn't a string problem. Every problem is a algebraic data structure problem. They can be parsed and transformed as a natural generalization of strings. And, I love Lisps but they are as stuck in the past as APL, I truly believe that in 1,000 runs of the simulation our world would come out bottom 10% on how naturally we manipulate macros. Macros look strapped on; we're living in a first draft.
The goal of any advanced programming language should be to expose the thought process as a form of algebra, so we can learn to fly. APL did this well for its time. This is the appeal of languages like Haskell. We have barely scratched the surface of the algebra of parsing combinators, as a potential backbone for programming.
My imagined successor to APL is in spirit only: It is so agile at implementing macros as monadic parsing of algebraic data types that all other problems become easy.