I've been doing some research, and I've actually found what I think is honestly quite a cool, but obscure civ, Sogdiana. This was an Iranian civilisation that reached maturity around the 7th century BC, and from around the 4th century AD to the 10th century AD, they dominated the Silk Road, they were involved in almost all the trade along the route, being the ethnic group inhabiting Bukhara and Samarkand (they founded those cities). In fact, chinese records (which bear in mind are biased) painted them as the archetypal medieval merchant, money grabbing, shrewd and drawn to profit, they allegedly educated their children from an early age on how to read and write so that they could fulfill commercial duties. Truly a fascinating bunch of fellows!
So, I've come up with a rough design.
Leader-Divashtich
Capital-Samarkand
UA-"The Silk Road" or "Road to the next town"
"Merchant specialists yield +X science and increase land trade route length from the city by 25%. Land trade routes yield +1 gold per 5 plots of the length of the route"
UU-"Sabao" A great merchant replacement, it's essentially a caravan chief, these guys organised trade in far off lands and negotiated deals, sometimes establishing permanent settlements or communities. Not sure what the bonuses would be, but, it would either be science or gold related, considering the theme of the civ.
UI-"Fortified market" (I will come up with a better name I swear!)
Unlocked at guilds, +50% defense and may not be built adjacent to another Fortified market. +2 gold, and +1 food if built adjacent to a source of fresh water. +Y science if a trade route passes over it.
The Silk road passed through the rugged and barren terrain of central asia and Transoxiana, and as such a series of market towns sprang up around the various Oasis' and pockets of fertility, these buildings were often fortified, and specially equipped with caravansaries and everything else a convoy might need. The Sogdian towns and cities were places where different cultures met and exchanged not only goods but artistic styles, ideas and philosophies.