I'd like to be able to do something like this (unless it's already possible, but I don't think so)

const PREFIX := "MyPrefix";
const SUFFIX := "MySuffix";
const MY_CONST := PREFIX + "_" + SUFFIX; // equals "MyPrefix_MySuffix"

You can already do that..
What you can't do is assign an array, dictionary, or other 'dynamic' type into a constant.

Ok, I'm officially a noob
By the way, declaring a constant as an array of constants would be a cool feature as well