Who were the parents of Princess Celestia and Princess Luna. Here's my (very, very, very wild) guess: Star Swirl the Bearded was their father and Baltimare was their mother.
Baltimare was an ordinary female unicorn with average magic, but of course Star Swirl was the most powerful male unicorn in the history of Equestria, which would explain why Celestia and Luna were Alicorns, not mere unicorns.
The city of Baltimare was founded by Celestia and Luna and named in honor of their mother.
In addition, Star Swirl and Baltimare adopted a young orphaned male or female unicorn, from whom Prince Blueblood is descended, which would explain why Blueblood is referred to as Celestia's nephew in The Ticket Master.