I think it would be cool if a language had a parser generator built in so that you could write BNF grammars right in the source code and the compiler would be able to turn these into parsing functions (this would also be really easy if the grammars were interpreted as Parsing Expression Grammars). What would be even better is if you could write text in the languages these grammars describe and have the compiler compile them right into the program alongside the code generated from the language itself.
For example, if you were writing an assembler, you might define a language for defining mnemonics and how they (and their arguments) translate into opcodes. Then you would add text in this language describing the architecture that your assembler will target. Rather than this parser running every time you ran your assembler, the output of the parser could be embedded right into the program so the result is highly efficient.
An efficient code generator that could be called at runtime would also be useful. This would make applications like formula-driven fractal generators, graphing programs, electrical circuit simulators and numerical analysis packages fast and easy to write. Combined with the parser generators, it would also make it very simple to write a compiler for this hypothetical language - just define the syntax and the semantics, plug in the code generator and there's your compiler. It would be almost like having a compiled homoiconic language, except with a proper built-in syntax (unlike Lisp). Unfortunately it would compete with itself so is unlikely ever to be written as a commercial compiler.
[...] be desirable to be able to use arbitrary strings as identifiers. For example, suppose that we have a language that has a parser generator built in. Suppose also that we give it a parsing rule like [...]
[...] we're all using languages which can compile code at runtime it's going to be more important to think about when your code is being compiled. Take a fractal [...]
[...] than one meaning. One is that ALFE itself can be used for any purpose. Another is that (via the built in parser generator and code generator) you can use it to create powerful domain-specific [...]
That's called Lisp.
Kidding, but you're describing something that's a very Lisp-y ideal. The parsers would be more powerful, of course, since you're able to define the read stage (tokenization).