> The good thing about this is ease of debugging.
Exactly!  +1 for the idea.

> On the other hand, the repr of a value might be very long:
You can restrict the length with % formats.

> Also, all values don't have a very informal repr:
Not your problem.  This change will still be much more useful than the current 'x'.  Some reprs are very helpful, other ones give the ID so can be used in debugging, other ones are not helpful at all so authors will have to make them more helpful or debug their code otherwise.