LeHam
Prince
I always preferred the civ II trade route system to that of the later civ games, since it actually meant you could play civ as a builder, trader or militaristically. It's probably not a great idea to basically copy/paste the system to civ V, but it is a great idea to update it.
How cool would it be to see important trade routes appear on the map ? Auto-caravans every 3-4 turns that can be destroyed by others (raiding trade routes for gold, at the expense of the trading nation). You'd have to actually weigh up whether it's better to trade with far away civs for increased risk of raids but better rewards or your neighbors for less raider risks but less reward...
Oh, yes, also agree with this trade routes idea you have. Now we can just "blockade" trade routes, but it would be fun if we could raid the caravans, and have to protect ours from other raiders. A new way of dealing with barbarians, perhaps.