The city of Ilata.

I suggest designing the city's appearance in the style of Machu Picchu, the ancient Inca city, and the inhabitants will be humans like those from the Order faction. I think it would be unusual and original.
I haven't decided on the name and racial ability yet.

Perhaps the Sanctuaries of Silat could be made in the Inca style, as you suggested, and there would be small villages-fortresses of ordinary citizens of the Free Cities nearby (because you wanted to connect Silat with the Holy Cities, right?).

And one more thing – how do you like the idea of replacing the air elementals with air riders to avoid repetition?

I'm sure that if there was a Free Cities faction in Heroes, it would include Free Mages.

But where can we add them, who can we replace? Maybe, if we remove the air elementals, we can place them on the third level, and move the priests to the fourth? And all the other units would be servants of Silat. What do you think?