My opinion is to hire a good lawyer.  Brad can correct me if I'm wrong, but I think it's Ubisoft that holds the rights to nearly all the old Microprose titles, including the original Majesty.  (Obviously not the Civ titles, because Meier got that written out of the deal when the company's assets were sold.)  The current license holder sadly is not keen on words like "sharing," and "community," and I think the phrase "Open Source" isn't even in their language.

You're more likely to get results if you simply find some way to mod the existing code, much as the Master of Magic modders are doing over at the Realms Beyond Forums.  And I sincerely wish you luck, because I thought the original Majesty had some great ideas that weren't sufficiently pursued, while Majesty II was a disappointment.

Good catch.  But as Paradox hasn't made any of its games Open Source, I can't see them doing it to Majesty.  You could always ask them, however.  Best of luck.

Obviously you couldn't actually call it Majesty. But a game that plays similarly. The spiritual successor comment is probably the closest to what I was thinking of.