I have added it to the roadmap.
We appreciate it Frog, and we appreciate you.
Maybe you should have to have a Diplomat in order to activate access to additional info. That would add a reason to use Diplomats.
This isn't a bad idea insofar as ideas go. However, 'Espionage' isn't in the game yet. If you recall that this was in an Expansion (pretty sure it wasn't just DLC) for GC3 - so I would expect this type of thing to be addressed in the espionage system, rather than doing it via diplos now, and then having to redo it with spies later (this is also more confusing).
So I think that assuming that we're just going to wait for additional systems in this space is probably a safe bet.
Having said that - having a diplo overlay /does/ make sense now though, because already have access to the information - it just doesn't really make sense to have to click on all other of the 17 civs just to see who they're at war with.