Yeah, telegraphy could be so awesome. Terrestrial telegraphy is one thing, and could be handled with a single tech, upgrading roads with telegraph-poles - for the sake of simplicity. Now submarine cables are way more interessting! First, you'd need access to gutta-percha (very rare) for the insulation, later some other plants could appear (perhaps with random events; new plant suitable for latex found - which happend with the bullet tree) enabling other civilisations to lay cables... until the event of synthetics/plastics...
This could add so much to gameplay:
- national/terrestrial communication-networks could be either build by private enterprises (linking the valuable cities automatically) or you could built a state-network (where every city is linked, or sth.) handled per civics?
- for submarine cables: you should be allowed to plan the routes and lay them automatically by ships (with increasing distance), early cable-laying attempts are bound to fail at high %, but the more are layed, the better the success-rate.
- Again: submarine cables should be laid either by state/planned by you and there should be corporations/great engineers that will lay them automatically (like spreading), but only where economics flourish.
- there should be the gutta-percha resource, without you can't lay early submarine cables (or pay much more for it!)
- there should be cable-ships, repairing such cables. There should be possibilities to cut those cables or to wire-tap (spionage!).. those cables are bound to be important up into the modern/future age!
- Overall, telecommunications-systems should be totally overworked: from early optical signaling (fires, etc. way not only for certain civs/or bound to wonders) to messengers (horses, ...) to carrier pigeons (bonus-tech? not every civilization should use all system of communication..) to optical systems/telegraphy (semaphore, ...) to the electric telegraph... wireless telegraphy, telephones, submarine cables, and so on...
Now each of those communication-system should drastically alter your possibilities of commanding your troops and colonies. Therefore, the better the communication, the better the `fog-of-war`/informations on the map or simply spionage-information. Also there should be a delay in commanding your colonies, if there is no direct communication. And if the barbarians have plundered one of your colonies far away, you shouldn't be able to do something about it, until it's too late.. something.. I don't know how this could be done, without restricting the gameplay toomuch.
This should also affect the dimplomatic possibilities. Beeing in direct contact from leader to leader instantly from the start is just silly. Working out contracts should take some time/turns.
Also there should be the possibility for censorship of foreign messages (why not allow telegraphy for military only, open it to the public with other effects, etc..) There should be a need to have your own cables. What about cable-message-forgery if you are in the position to do so?
Also: telegraphy enabled the first usefull weather-forecasts: no more crop failure (random events) due to a unforseen snowstorm, ... also all the ships are way more safer now.
Time can be distributed (from observatories) and synchronized. There could be diplomatics about timezones, etc.
There should be telegraph conventions and a bureau handling international deals (dropping tariffs, deciding on apparatus/codes to use).. and here: only those states can participate/vote which have a national system (eg. the US had a private telegraph system and was not allowed to vote)
This whole field could add so much to civ!
Fun should trump reality for all instances 
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