>> It's not just that particular use case that I have in mind. As I wrote
>> earlier, the feature I have in mind should not be an additional (minor
>> or major) mode, but a core feature [...]
> I'd recommend to aim lower at first. The modes and packages are a big
> help for people not to step on each other's toes. Trying something new
> with the approach "I think everyone should have it" is a tall order,
> especially with a community as diverse as Emacs.
I understand your viewpoint. I'm proposing something, I'm not ordering
anything. It just happens that this idea cannot be implemented with a
mode or a package, like proportional fonts for example. Making this
possible would not change anything for users who would not want to use it.
If it is not possible to implement this, or if nobody else thinks it would
be useful, that's fine.