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It has been mentioned before I think, but hear me out.
In Chapter three, page five (clickie), Arthus and Camelot get mentioned by name. We know for a fact by know that Nanatsu no Taizai draws inspiration from old English mythology, with the typical freedoms every Mangaka takes, so why not incorporate Arthus and Excalibur into this? Especially if one of the Sins is (apparently) named Merlin.
For what I see, the Sins may very well become somewhat involved in the legend from the Knights of the Round Table in one way or another. Be it with Merlin as the advisor people know from the tale, Gowther taking the spot of pure and noble Galahad or maybe even Ban becoming somewhat of the Green Knight, there are quiet a few possibilities. Maybe they even end up being part of Arthus' entourage. Nothing seems to point toward this so far, I admit, but Nanatsu still is relatively young and I've seen stranger things being pulled off after 100+ chapters of story. Also, I'd pay a whole lot of money for Excalibur to appear in Nanatsu no Taizai.