>
> Suppose we reserve one key for third-party packages.  Let's refer to 
> that key as C-α, to avoid specifying which key is chosen.
>
> Various packages will set up their own bindings for it.  If you load 
> more than one such package, which package's bindings will you get? How 
> do you get the ones you want?
>

This is something that should be left to packages.  Org-mode would bind, 
say, C-α a, C-α c and C-α l, or perhaps C-α o a, C-α o c and C-α o l; 
Magit would bind C-α g and C-α M-g, or perhaps C-α g g and C-α g f; and so 
forth.  There will be conflicts, of course, but only occasionally, and in 
those cases users would have to do something to resolve the conflict.

>
> Here's an idea.
>
> Have one command you can use to specify which package's bindings C-α 
> will run.  It could be C-α C-α; then individual packages will not give 
> their own bindings for C-α C-α.
>
> So you can type C-α C-α Foopkg RET, then C-α gives you the Foopkg 
> bindings.
>

I'm not sure I understand what you mean.  Would this mean that each time 
you want to use, say, Org-mode you would have to C-α C-α org-mode RET 
before typing C-α c?  If so, I don't think this would work.