What is the policy for moving a feature into core or not?

[ndame]

Let's take the kill ring. It's a central piece of emacs, yet I don't see any 
keyboard
based solution built in to browse and search the kill ring. M-y is extremely 
basic. There is
a menu, but it's mouse based, inefficient.

There are external packages, of course, but I wonder if there should be a 
builtin way to
navigate and search the kill ring from the keyboard. By builtin I mean a 
package available
from at least elpa.

Is there a current policy which governs what features are integrated into the 
core (elpa) 
and what features are left to outside developers?