So, I definitely think that you should do Auckland!! New Zealand cities are cool, too! This is totally not flattery (okay, sort of...) but seriously, choose a city from your home country and do it right!
But I'm also going to suggest the Tri-Cities, Washington. There are three major rivers that join up here. The biggest one is the Columbia, which conveniently makes a 90-degree turn after skirting by one of the three cities. The small Yakima River splits two of the cities up, and the bigger Columbia separates both of those from Pasco. Pasco is conveniently bounded on the east end by the large Snake River, but on its east side there are some smaller communities (nice to add at the end of the game for one last burst of difficulty--you had such a great system, and now you have to connect to these stations across the river!).
Especially if the growth followed the communities--Pasco was the oldest, then Kennewick across the river, both on the far east side of their current boundaries. Then decades later, a population explosion happened in Richland as a nuclear project was developing. Slowly, Kennewick creeped west to join Richland (and then Pasco did the same a couple decades later), but for a long time there was a lot of empty space separating the two old cities from the heavily populated Richland--it'd be a nice change of pace! Then the coolest bit (besides the small communities on the other sides of the rivers) is how both Kennewick and Pasco are bigger now than Richland.
I can see it now: starting on the east side of the map, on both sides of the river, but then about week 2, a whole bunch of spaces (including a couple specials in the north) pop up WAY west of where the player begins. Tunnel management becomes critical--do you connect Kennewick and Pasco to each other and to the new stuff in Richland, or do you send all Pasco traffic to Richland through Kennewick (which was the real-life solution for 30 or so years)? Then Kennewick slowly creeps westward, then Pasco finally follows afterwards. (Pasco is smaller but has both the college and the airprort--Kennewick is the biggest but doesn't have anything super "special" besides the mall in the west.) Interested?
One last bonus: there's maybe a quarter of a million people in the area. That's small enough that if you make them one of the cities, people there will go CRAZY and buy like CRAZY... but it's big enough to have a substantial potential customer base. I mean, L.A. is great, but how much of an impact will it have on sales if you select it? (Nothing against it, but people in L.A. are used to having their city used as a location for things. It's just not going to move units, I fear. I think doing a smaller city will likely motivate more local-pride purchases!)
Again, it's a phenomenal game!!