Okay, you want to keep it real, so no silly names to their faces (behind their backs a completely different matter, so long as you don't get caught), but remember that some royalty, emperors and even nobility have different titles.
Queen Victoria was Her Majesty in Britain and Ireland, but in other parts of the empire, she was Her Imperial Majesty. HRH Charles, Prince of Wales, is also the Duke of Cornwall, but in Scotland, his official senior title is HRH Charles, Duke of Rothesay. He also has lesser titles.
So, he might be His Imperial Highness, X, followed by 'lord of all he surveys' (or whatever the official flattery is, such as Defender of the Faith). But, depending upon where he is, and who's attending him, he might also be His Majesty, Hereditary Prince-Elect of the Faraway Islands, Lord of the Sceptred Isles.
Or, my sovereign.
Or, Bob, to his mother
(despite the name change to Ferdinando on his coronation).