That's an interesting use of mixins
I didn't go beyond code reuse and separating same class in multiple files.
I have just read your ideas page.
I thought i should make a list myself.
1. Function pointers and function objects
2. Template objects with multiple subtypes
3. Inheritance from registered classes. Without this your whole program design changes, it sucks.
4. Closures/Anonymous functions/Lambdas. Functional programming is the best.
5. Builtin array, set and maps. Everyone needs them, why are they addons. They can still be optional, but builtin is a must. If they are builtin i can finally write some AngelScript only programs. I can write a 1000 word essay how important built-in containers are for a language.
6. Static functions. Great for organization.
7. Generic handle. like ref addon but supports all types and can implicitly convert.
8. Varidic functions. Or functions with argument array. void MyFunction(args). Would be easy with #7
9. Type comparison. if( typeof(my_int) == int ). This may even be a switch/select statement. Useless without #8
10. Unsafe keyword. Mark functions and objects to be free of garbage collection. Also warn user of the dangers. Maybe go crazy with pointers.
11. new and delete. Unmanaged memory for all types. ( would be double awesome with unsafe keyword)
12. Stateful functions. With a keyword like 'yield' . Pause execution, resume when called by other later. Script languages are great for stuff like this.
13. Reflection. No not as powerful as js or lua. At least functions like createClass .addMethod("func()") .invoke("func")
These are just stuff i like, no need for a feedback. You are likely very busy.