Moving content from an url to an instance of the same url. Pros and cons ? And best timing to do it?

Hello guys !

I would like to get your feedbacks on a possible url strategy, understand the risks associated with Google downgrading the website or not, and if there is a good timing to move content from a url to an instance of the website.

I am launching a user generated content website for cities.

Let's call it cities.com where I plan to have existing content from the get go on one city.

At the start I had planned to have only one city, on the main url.

However, after some discussions with people around me I discovered I could create niche instances of this website that could bring a better fit in terms of naming for search and also more targeted content.

For example:
Paris.cities.com
Newyork.cities.com
Beijing.cities.com

So it gets me wondering, shall I launch my first city with the instance already ?
paris.cities.com ?

Or can I launch with content on a normal url (cities.com) and move this content later to an instance without risks ?

Thanks !