I'd like to import some features from functional languages like OCaML and F#.
The main feature I'm proposing is the possibility of using typedef to define sum types. They can be thought of as type enumerations.
This will, in some cases, reduce the need for dynamic making for sounder typing.
For example json.decode would no longer return dynamic but JSON a sum type, something like:

typedef JSON is Map<String, JSON> | List<JSON> | Literal;

Sum types could power a new behaviour of switch case that resolves the type based on the typed enumerated. Kind of like assert and if does with contemporary is syntax.

typedef T is A | B | C;
...
void foo(T arg) {
  switch(arg) {
    case A: // here arg is casted to A
      break;
    case B: // here arg is casted to B
      break;
    case C: // here arg is casted to C
      break;
  }
}

A better syntax for this type of switch case might be of order, maybe something like dart-lang/sdk#57173.

typedef T is A | B | C;
...
void foo(T arg) {
  switch(arg) {
    case A -> ... // here arg is A
    case B -> ... // here arg is B
    case C -> ... // here arg is C
  }
}

This would be a powered down version of OCaML and F# match <arg> with as I've not included a proposition for type deconstruction, which would probably require tuples (or more in general product types) as discussed in dart-archive/linter#68.