Are Nameplates possible with the current API?
 

It seems to me a a lot of other people that ESO without Nameplates is Dull and that the Guilds have no representation and/or reason to show off since no one know what guild you are from. This would be very useful for PvP and Trials/Arena/Dungeons.

Is it possible to make a Nameplate addon that would give Name, Guild Name, HP Bar and so on? All of the info is there and used by the Unit Frame addons and I have seen a Colored Reticle addon do this. Can it be done with Nameplates and give us a more active social / guild aspect?

No, it is not possible. Addons can modify whole user interface like menus or unit frames, but they can't add anything to the 3D world.

The closest we can currently get is Social Indicators. It can only show you members of guilds you are part of and only add it to the unit frame.