That’s a gross oversimplification on headers. They’re a holdover from C, but they’re still a pretty core part of how the C++ compiler works. These days it has about as much to do with C as anything in C# has to do with Java. If they wanted to make header files optional they certainly could (and in some ways they are especially if you’re using tools to automatically generate headers/C++ files from unified files), the language specification changes every couple of years.
The is already there in unreal though. Whether or not it already exists in a different language is irrelevant, because it is already supported in the language the editor is written in. I don’t think it’s really worth the time to implement a new language to support features that are already supported.
There have been hybrid languages before. Even you mentioned managed C++. Few people use them because the performance is generally sub-par or the syntax is often even worse than dedicated languages (jack of all trades, of none), so using 2 languages for the parts of your application that benefit from the languages’ strong points is almost always better.