Since Charles and William have the legal power, on their assuming the kingship, to elevate their wives, Camilla and Kate, from the title of Princess Consort to Queen if they wished, then surely Harry also has this ability to so elevate Meghan to Queen. Even though he is currently 6th in line, and Meghan has not yet received UK citizenship, is a Queen Meghan a real possibility?! Stranger things have happened in British royal history!
The legal power of the monarch, such as it is in these circumstances, would in fact have to be used to facilitate the wish of the new Queen to not have that status. I suspect that some variant of an instrument of abdication would be required, followed by an Act - quite possibly 'Her Majesty the Queen Consort's Abdication Act' - whereby she renounced her title. Charles (or George VII as it still seems it might be) would then bestow the title of Princess Consort upon her. The alternative would be a follow on from the debate over the ending of male primogeniture and to use legislation to equalise the position of the sovereign's spouse so that they are either Prince Consort or Princess Consort, removing the variance which currently exists.
Or, he says optimistically, the Daily Mail and Daily Express could just grow the **** up about Diana and ignore the legions of vacuous cretins banging on about Saint Diana (the patron saint of causing England Rugby Captains' Divorces), etc, etc. If the Mail starts campaigning for a 'time for common sense: we need Queen Camilla' it'll mean Dacre is getting desperate for his knighthood.
So, if, by some odd series of events, Harry became King, then Meghan would become Queen upon his accession to the throne. Which would come with its own magnificence of a coroner having to report that a Mr Skylog appeared to have died of a simultaneous stroke, heart attack and attack of apoplexy...