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Thing is ... we are City, not Bayern, Juve or Chelsea and shouldn't try and copy their's but retain some sort of identity that is City throughout. The shield is something that has featured in all of our badges and we can argue that that is synonymous to City, like the Lion is to Chelsea, Stripes are to Juve and the Bavaria Flag of Bayern.
if you take the Shield away (which has 'floated' with more space around the sides in the past), to fill the whole circle as which I assume you're suggesting, it just looks like a fat circle shield instead with more empty space ...