First of all, this has been done in a normal game and NOT on a sandbox.
I wanted to test the possibilities of a residential only city, with no power, no water and no services, and try to understand the basic behaviour of the game.
The city is composed only by few high density roads and $ residential zones, nothing more.
Broken RCI rules
As many have noticed the RCI is currently broken.
The RCI bars indicate the need of workers/shops/jobs based on the current population needs. This can be considered OK.
However the RCI DOESN'T impact the amount of people wanting to join your city.
Building only residential areas, my Residential needs are now the lowest possible. However as much as I zone, new citizen will come.
Moreover, the speed at which they come is fixed and not influenced by RCI or even happiness.
If you have any free space in your city, new people will come. RCI doesn't matter, happiness doesn't matter, power/water/services doesn't matter.
They will always come at the same fixed rated.
Broken happinness rules
To keep your people happy some needs must be satisfied.
Some needs increase the happiness, some decrease it.
If you really satisfy one of the need increasing happiness, doesn't matter how much some other things decrease it, you can still be positive.
I'll try to be more clear with an example.
I have no power, no water and not any services in my city, then happiness is really negative.
However lowering taxes, makes people happy.
So if you lower the taxes enough, you can still make people happy doesn't matter how much unhappinness you generate on the other side.
With my broken city and low taxes, i can have full happiness, and a global hapinness rate varying between 75% and 80%.
This will allow all your building to grow to high density even in the worth living conditions.
Global happiness rate
Don't people leave ? Don't you have any trouble ?
Of course people leave my city because it's so bad.
Nobody leave because of no job, no shop or no money. This make them slightly unhappy but these factors are not important at all.
The amount of people leaving because of no power and no water is very low and can be ignored. water and power are not so important either.
Reaching around 400k people, health and then crime become a problem. A lot of people die and leave because of these very bad conditions.
However there is so many new citizen wanting to come to my city (even with the lowest RCI possible) that people leaving is not a problem and is compensated with the new incoming ones.
Keep in mind that new citizen will always come to your city doesn't matter what you do.
The important thing is to make sure there is at least more people incoming than leaving one.
The more people i have, the more troubles i have, the more people leave.
Then the growth rate slightly decrease with time. But the stupid RCI will always replenish my population, keeping my city growing.
Other problem encountered
The only major problem encountered is fire.
With no education, no police and no fireman, fire is a real problem.
Moreover each time a building is in fire it will propagate to next ones. In such a case, most of your city will be burning.
Fire can really stop your city from growing.
To solve this problem, i had to create a foreign city to send me some fireman. This is the only service provided to my main city.
Currently i have 5 fireman solving all the troubles.
500k people ? what's about traffic ?
with no job, no shop and nowhere interesting to go, people have no reason to be in the streets.
My streets are close to empty. Only people in the street are homeless (a lot of them because of people leaving), new incoming citizens, and a lot of criminals going to do any criminal acts.
traffic is not a trouble with such a ghost city.
My current population is a bit over 500 000 citizens using roughtly 30% of the map.
With more time it could probably easily reach 1 or even 2 million. However this process need a lot of time so i didn't increased it more than that.
On top of all know bug such as pathfinding, traffic and others, basic simcity rules as RCI and happiness are currently so broken that the game is meaningless.
Basically you can do what you want, you can't fail a city.
Keeping a city growing is automatic and not based on how successful your city is.