What would break if class was merged with struct

[Ola Fosheim Grøstad via Digitalmars-d]

I wonder, what would break if all the features of class was 
merged into struct?

Imagine that:
    class Something ... { ... }

is lowered into:

    struct _class_Something ... { ... }
    alias Something = MagicClassRef!_class_Something;

Is it conceivable with some language changes, a bit of automated 
source updating and a little bit of breakage?