The latest reports all trace back to a comment from Eduard Pogea, a respected German tuner, who apparently spoke with this "reliable source" and got the scoop. It's been circulating around the internet for a few days now.
Problem I have, is getting my head around the words "reliable", "information", and "Alfa Romeo" all being used in the same sentence!
The 6C has long been rumoured (since 2014 at least), and most peg it as coming in as a sportier version of the Maserati Alfieri. That car has been shown for 5 years now, but production is delayed to 2020 according to latest reports. This would mesh with the latest rumour, that a 6C would also come out in 2020.
AFAIK, the Alfieri was developed independently of the Giorgio platform that underpins Giulia and Stelvio. Whether Masterati ditches their car's design in favour of adopting cousin Alfa's platform or not, I wouldn't know. If they do, then it will not have the carbon fibre structure that makes the 4C such a lightweight. Instead, it will be a sexy, bulky GT car in line with the hefty Giulia. Still, stick a good drive train in, and proper suspension, and it will be sharp. But look for it to cost north of US$100,000. There is no reason to come in lighter.