I agree with this 100%. Keeping with the original suggestion, please tell me what you think of this idea:
Instead of an 'item' that allows trading long distances, how about a city building that allows this?
Call it a "Shrine of Teleportation" or something along those lines. This building could be made available via an advanced/high-level unlock (for sake of balance) on the 'Magic' side of the tech tree. Once researched, you must build this structure in at least two cities. Once built, units (or stacks of units) can teleport themselves to any other city this structure is built in (for trading items, or just simply for warping vast distances). Additionally, it could also function as a trading station, where any units stationed in a city with this building can automatically trade items as if they were standing in the same place, but in fact they're in completely different cities. Okay... I really, REALLY like this idea now. 
If something like this can't be modded, I truly hope this (or something similar involving teleporting) gets considered for an upcoming patch. I can't imagine the possibilities that would open up for players and AI opponents alike if they could correctly utilize something like this. I can even see a potential diplomacy option to allow these buildings to transport units between cities of different factions, once it is proposed via diplomacy and the other side agrees to grant access (once both sides have researched and built them). It would be more commonly used in magic-heavy factions, which is perfect IMO. Not to mention it would probably make the game far more enjoyable, and/or potentially challenging.
