PROPOSAL: Repurpose one key and reserve it for third-party packages

[Gregory Heytings <gregory@heytings.org>]

[S Boucher apparently intended to reply to the following proposal sent on 
emacs-devel.]

=Proposal=

It is proposed to repurpose one key, and to reserve it in the key binding 
conventions for third-party packages.  The keys that could be reserved for 
that purpose are:

Option 1. C-z, with a single exception: "C-z C-z" would be bound to 
"suspend-frame"

Option 2. C-z and M-z, with two exceptions: "C-z C-z" would be bound to 
"suspend-frame", and "M-z M-z" to "zap-to-char"

Option 3. C-o, with a single exception: "C-o C-o" would be bound to 
"open-line"

Option 4. C-o and M-o, with two exceptions: "C-o C-o" would be bound to 
"open-line", and "M-o M-o" to "facemenu-keymap"

=Rationale=

The current key binding conventions (see `(elisp) Key Binding 
Conventions') reserve keys for users, for major modes and for minor modes, 
but not for third-party packages [1].

When such packages need to bind a command to a key, they can (1) either 
suggest users to bind it to a key reserved for users (for example, 
org-mode suggests to globally bind "C-c c" to org-capture), or (2) bind it 
to a key currently unused by Emacs (for example, Magit binds "C-x g" to 
magit-status in buffers visiting a file in a Git repository).

Neither of these solutions are optimal: (1) requires an explicit 
configuration by the user, something which might confuse newcomers, and 
which other users might not want to do because they already use the keys 
reserved for users for other purposes, and (2) might conflict with the 
evolution of Emacs when one or more commands are bound to a yet unused 
key.

Reserving one key for third-party packages solves the above problems: 
third-party packages can automatically bind a few keys in that reserved 
area, without conflicting with keys reserved for users and without 
conflicting with future Emacs evolutions.

=Limit=

Conflicts are still possible, when two or more packages bind the same 
keys.  These are, however, conflicts between packages, not between a 
package and Emacs, or between a package and users' personal 
configurations.

Such conflicts are also less likely for typical users, who install a few 
packages each binding a few keys.

Finally, such conflicts can be dealt with without confusing users too 
much: a package could automatically choose fallback key bindings when the 
preferred ones are already used by another package, and/or issue a warning 
to the user that they need to bind its commands manually.

=Note=

[1] These conventions were written 25 years ago, at a time when there were 
far fewer third-party packages, and have not changed substantially since 
them.