I would suggest as another possibility not an heir to a throne, but a mighty monarch who might have died with thousands of others, some of them probably important people, in a sea disaster.
Musa II, Mansa of the Malian Empire, or Mansa Musa, is famous as one of the richest men in history. He ruled from about 1312 to 1337, and his pilgrimage to Mecca in 1324 and 1325 with an immense retinue and spending vast amounts of gold dust is legendary.
Apparently Musa II became Mansa by being selected as the deputy of the previous Mansa when the previous Mansa, Abu Bekr II, went on a journey. The story that Musa II is said to have told in Cairo is:
The ruler who preceded me did not believe that it was impossible to reach the extremity of the ocean that encircles the earth (meaning Atlantic), and wanted to reach that (end) and obstinately persisted in the design. So he equipped two hundred boats full of men, as many others full of gold, water and victuals sufficient enough for several years. He ordered the chief (admiral) not to return until they had reached the extremity of the ocean, or if they had exhausted the provisions and the water. They set out. Their absence extended over a long period, and, at last, only one boat returned. On our questioning, the captain said: 'Prince, we have navigated for a long time, until we saw in the midst of the ocean as if a big river was flowing violently. My boat was the last one; others were ahead of me. As soon as any of them reached this place, it drowned in the whirlpool and never came out. I sailed backwards to escape this current.' But the Sultan would not believe him. He ordered two thousand boats to be equipped for him and for his men, and one thousand more for water and victuals. Then he conferred on me the regency during his absence, and departed with his men on the ocean trip, never to return nor to give a sign of life.
Mohammed Hamidullah. "Echos of What Lies Behind the 'Ocean of Fogs' in Muslim Historical Narratives". Muslim Heritage. Retrieved 27 June 2015. (Quoting from Al-Umari 1927, q.v.)2
So possibly the mighty ruler Abu Bekr II of Mali died in a sea disaster, which might be considered even more important that the deaths of Margaret the Maid of Norway, Prince William of England, or Jan William Friso.