In another forum, I was responding to the issue of Speechcraft and mercantile being useless in game, and I think it would be quite nice if these things got included into TES V (highly unlikely).
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There should be an economy. This would make living as a merchant much more profitable than a dungeon looter, most likely. It could be somewhat like this - in Anvil, say, silk is cheaper than anywhere else, because there's more of it comming, from other provinces and such. However, in Cheydinhal there's a need for silk. So, if you bouth silk at Anvil and sold it off at Cheydinhal for a higher price - you've got profit. Plus, there should be the options to buy exotic materials through the means of caravans - like, there would be caravans going from Cheydinhal to Morrowind every three months. You could add your own goods to trade, and either order them to be bartered into other stuff which cannot be found in Cyrodiil, or sold. Obviously, you could buy things also.
Eventually, it could maybe be possible to build a ship for yourself, and tax the other merchants who wanted to deliver goods with it, also deliver your own goods (of course, you'd have to pay taxes yourself, as well as payig the crew and the captain, too). Eventually, it could all turn into a large, well-oiled trading empire that brought hundreds of thousands of Septims, also making you a very famous person!
On the other hand, smuggling could be an option too. It would be really risky, though maybe more profitable. Also, whereas honest trading would make you famous, smuggling would probably obviously make you infamous.
Also, maybe it would be possible to open up an inn, which would require both speechcraft (to attract customers and workers) and mercantile (obviously to do business).
Also, to make speechcraft skill better, maybe there should be the option to get into the court of one of the cities, and make your way up to becoming a diplomat, or maybe a herald of the Count of that city. You would be sent with messages, also with secret tasks such as to bribe people in the court of a rival city, which would require high speechcraft.