Can you comment in generalities on the scope of changes to LT's behaviour end users can expect to be possible by design, e.g. the entire stack is modifiable to

Can you comment in generalities on the scope of changes to LT's behaviour end users can expect to be possible by design, e.g. the entire stack is modifiable to the extent that Emacs (even the underlying elisp interpreter) is modifiable, only via a limited API like Sublime Text 2 has (which results in end users being reliant on major changes coming from the top down), or something else entirely?