I would have difficult to find a better flag to Brazil or my state, but for my city, surprisingly, it was very fast. First, let's how the current flag looks like:
The first, obvious, try, was remove the coat of arms in the center:
I'm certain the combination of Order of Christ's cross (a Portuguese Templar branch) and the Nordic cross would be a pretty unique flag, but I think I could do something better.
The São Paulo coat of arms depicts a flag with the Order of Christ's cross, similar to flag above, in reference to Portuguese colonization and the city foundation by Jesuit priests. But, usually, the cross is incorrectly depicted in city coat of arms (see this image of a true bus). As Cananeia, a city not so far São Paulo, uses a (almost) correct Order of Christ's cross, São Paulo should use that wrong depiction as an exclusive mark. This is my final design, with that depiction:
I like mainly the grey shade, proper to a city of about 11 million inhabitants. It can be easily adapted to other uses, as department logos. Looks the following example, with a green color to the "Municipal Secretary of the Green and the Environment":
I liked, but I'm biased about my city, so your opinion is important!
Please, comment; it's easy and you don't need a Google account.
I think January will be a month of many posts; wait!