Any plans to introduce similar restricted districts to other civs? I'm not sure which ones would make thematic sense, or who might need to have their power reined in a bit. The Cree seem crazy powerful from my games with them, FWIW, as does Korea.
I haven't thought about it too much, although when I was looking at how to make Korea work differently it was a thought. Mainly the reason that got applied specifically to Arabia is because I love playing Arabia, so they got the most changes that fit with what I wanted to play.

Also, at the time that ability was first written, Trade Routes were not yet limited to either a Commercial Zone or Harbor, you got a TR from both. So some of the way the ability unfolded had to do with how the game played at that time. The first draft of Arabia was actually not a big hit--they didn't originally get +5 Gold from having both a Holy Site and Campus, nor did they get +3 Gold per city following their religion. That evolved in the subsequent patches.
In general when I design for this mod I've tried to stick to a "lopsided" design scheme where each civ feels substantially different from the others, both when you play as them, and when you encounter them in the game. With Arabia, limiting the Great Merchant has implications both for you if you are playing but also if they are your opponent. Since they get to "steal" the last Great Prophet it seemed somehow fitting that they might not get to participate in other GP races. The flip side of this is when you invade Arabian cities you're likely to end up with a bunch of Holy Sites and Campuses but little in the way to finance your invasion.
This same design principal, by the way, is Brazil gets their Brazilwood Camps from a Project, and the Aztecs have a unique National Wonder all to themselves. I've tried to think of how playing with the mod turned on could make a player (or me

) feel like they've stepped away from the core game and are experiencing something new, where they can try out some strategies that normally wouldn't make sense. That's also the reasoning behind Friendship bonuses and the new Secret Agenda flavors.