The old regional flag, adopted in 1991:
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The new design (there are, actually, five other official chromatic variants):
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The rationale for the flag change is, actually, kind simple: the previous flag contained the old logo, the current contains the new. The current logo has a heart, representing the openness and hospitality of its people, and the fact that Brussels is in the heart of Belgium and Europe (hosting many European Union's bodies).
The yellow iris is a symbol of Senne's valley due to a legend telling that the Duke of Brabant once won a battle because he knew that yellow irises only grow in shallow waters. It's fair to keep them on a flag for Brussels-Capital region.
I'm not fine with the design of the iris changing every time a flag is changed, so I thought about fixing a "classic" iris on the flag. Here, a stylized iris i.e. a fleur-de-lis:
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Or, alternatively, a more "natural" iris:
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I don't really think that the white fimbriation is needed (it appears in the four flags above), but it increases the distinctiveness of the flags. I kept the blue background as it remembers the EU flag, linked to the region.
Comments and suggestions are welcome.
You can read more about Brussel's rebranding here.

The new design (there are, actually, five other official chromatic variants):

The rationale for the flag change is, actually, kind simple: the previous flag contained the old logo, the current contains the new. The current logo has a heart, representing the openness and hospitality of its people, and the fact that Brussels is in the heart of Belgium and Europe (hosting many European Union's bodies).
The yellow iris is a symbol of Senne's valley due to a legend telling that the Duke of Brabant once won a battle because he knew that yellow irises only grow in shallow waters. It's fair to keep them on a flag for Brussels-Capital region.
I'm not fine with the design of the iris changing every time a flag is changed, so I thought about fixing a "classic" iris on the flag. Here, a stylized iris i.e. a fleur-de-lis:

Or, alternatively, a more "natural" iris:

I don't really think that the white fimbriation is needed (it appears in the four flags above), but it increases the distinctiveness of the flags. I kept the blue background as it remembers the EU flag, linked to the region.
Comments and suggestions are welcome.
You can read more about Brussel's rebranding here.