This is a good idea. Linking related issues/discussions:

#8439
#6374

I want to argue that this is different to default values in sense that usually const variables represent compile time constant values.

Arguably, one of the best feature Flatbuffer has is support for structs which represent fixed size struct with no vtable, and having compile time const variables would make declaring such structs with arrays much easier and readable.
eg):

struct EmbeddedMsg{
  v: [float:COMPILE_TIME_CONST_SIZE];
}

I want COMPILE_TIME_CONST_SIZE to be guaranteed to be compile time typed variable. I know that most people use enums for this but, kinda want to make it more explicit.

Also, when dealing with structs, sometimes you want to describe more complex relationship between const variables:

const NUMBER_OF_SENSORS: uint8 = 10;
const SAMPLE_FREQ: uint8 = 10;
const DATA_PER_SEC: uint8 = NUMBER_OF_SENSORS  *  SAMPLE_FREQ;

struct SensorData{
  sensor_status: [uint8:NUMBER_OF_SENSORS];
  sensor_data: [float:DATA_PER_SEC];
}

Any opinions? @aardappel

I agree having constants be declarable in schemas, usable within schemas, and output in generated code for all languages would be a great feature to have.