I think these cities usually show up in the game already to the best of my knowledge.
EDIT: I am so sorry, I thought the previous post was talking about the Kurds, not the Kushans...
I must have some kind of brain/reading comprehension problem and I apologize.
It seems to me that within the game the Kushans would fill the role of a pre-Islamic Pakistan/Tarim Basin civ which comes with its pros and cons, like the Phoenicians and the Turks I think it would be possible to simulate migrations from one core to another. However my biggest problem with a Kushan Civ is its longevity, it seems the empire fragments after a century, and much of its culture is already gone by 380. If a civ was to be made I think RFCA (Rhyes and Fall of Asia) would be great to consult with its Tocharian Civilization
EDIT: I just read the OP again, it reminded me that the Kushans are in fact going to be included in the game
Right now the Kushans are represented as barbarians which is good because it simulates the problems states in the region faced, the Indo-Greeks were overrun after all but also problematic because I don't think barbarian-controlled cities participate in trade. In reality, we all know that the Kushans was a primary beneficiary of the Silk Road.
Indeed independent cities could help, and Taxila could be one of them.
TLDR: Sorry everyone I confused the Kushans/Kurds because I had not slept enough and wrote in error, but there is potential for a Kushan Civ if its development is stretched to the more distant past.