I would very much like to see the full Coronation cockade worn again at a Coronation (it was worn by Queen Alexandra in 1902 and Queen Mary in 1911 in its full setting, and by Queen Elizabeth in 1937, only with the central cockade).
Above or below the Coronation cockade, Camilla could possibly add the Koh-I-Noor, mounted again into a brooch frame, possibly with the Lahore diamond and the Coronation earrings mounted as pendants to the brooch.
And to add the regal "final touch", I would suggest that Camilla adds the large King William IV's diamond brooch, either on one side of her cleavage (like Queen Elizabeth did in 1937, see picture below), or lower on her bosom (below the Koh-I-Noor brooch and the Coronation cockade).
Such large stomacher and brooches, all parts of the Crown's heirlooms and studded with large historical diamonds, would tremendously highlight Camilla's regal status as Queen consort. Perfectly fitting for a Coronation!
If my suggestion above was not retained, then Queen Mary's diamond stomacher could be a nice and stunning alternative for Camilla, and would be a perfect stylistic match with the Durbar tiara (which I have suggested as my favorite option for the head ornament):