CIV Total War

[caravel]

without having to make sure every turn that you've not forgotten again a diplomat in the middle of North Africa....

Well they could have fixed that by improving the campaign map AI and having diplomats that actually use a bit of common sense. A diplomat that has been inactive for a set period of time without a mission should simply warn you that he's on his way home again as he's no business waiting about in the middle of nowhere. If you wanted him to stay put for whatever reason you could merely set him to "hold his position". That way you wouldn't have to worry about losing/forgetting about him as he'd come home eventually anyway. It's these sort of oversights that equal masses of irritating micromanagement.

Also the trade fleets that buzz around everywhere on the RTW map would definitely count as a transportation network for emissaries and other agents, thus they should be able to simply hop from one port to the next and not need to be brought to their destination in one of your own naval vessels. It's hardly fitting for an assassin or spy to be dropped off by one of your own faction's ships, when they can utilise the trading network of merchant fleets to get where they're going much faster and much more stealthily. They would either buy or work their passage and enter enemy lands much more stealthily than being dropped off by a naval warship on a dedicated mission bearing your faction's colours, which is nothing short of ridiculous. Agents should be able to travel much further than armies on land also, as an army would move a lot more slowly take a lot of preparation, equipping, organising and would be restricted by it's slowest foot units. A lone agent could take shortcuts through the hills, or through other impassable terrain that a large army would not venture through. The agent may board a ship near the coast for part of the journey, may be on horseback or in a carriage for another part and may utilise other waterways.

In short an agent would travel much faster than an army, would not need military transportation, relying instead on merchant shipping, and would be able to return home or ask for orders if he has been forgotten or mislaid.