A few suggestions
Hi I'm new posting in forums , but I've been playing Civ form Civ II and think can add some ideas to dicuss. The first one is trade.
Definitively it is too simple to deal with the inbounds. Happiness & healthy. Trade is automatic, no revolts, no crime...
I think trade could work in a more "playable" way and won't be difficult to develop: it could be there units for trade like galleys or wagons, so that they are ordered to do a certain trade route. The income for the treaure happens when they get to a city and the amount depends on de suitability of both cities (start and end) for the trading, just the way Civ IV sets the automatic trading. Theese units may be destroyed/captured by enemies or get robbed in the peace times. Corporations may also get boosted with this trade way. There may be civics that will allow international trading and other that may prevent from trading outbounds. This way of trading may force players to secure trade routes on land or seas if they want revenues from their citie's trade.
This way of trading could lead to piracy and smuggling. You could build hidden nation ships to capture traders or promise Rome not to trade with Carthage but continue doing it by smuggling.
I think it's not difficult to develop because great merchants in Civ IV do trade missions almost the way the whole trade could work in Civ V.
I'll apreciate your replies.
More ideas to come soon...