Who would become king if the king died with no heir in medieval times?

 It depends on the system followed by the dynasty and the political situation prevailed at that time.

Let us review, few cases:

1. Raja Raja Chozhan has to wait, though he is an eligible hair, after his elder brother Aditya Karikalan’s (Prince) untimely death. Due to political turmoil, his father’s brother Sundara Chozhan become the emperor. After his death, with an agreement made earlier Raja Raja Chozhan become the emperor instead of Sundara Chozhan’s son. The waiting period helped Raja Raja Chozhan to prepare himself as a great emperor of Chozhas spanning his rule beyond Indian Peninsula to South East Asia.
2. Due to various reasons, no hair / death of eligible heir, Vijayanagara Emperor was ruled successively by four brothers.

• Immadi Narasa
• Vira Narasimha
• Krishna Devaraya
• Achuta Devaraya

We know, Krishna Devaraya was the great ruler and Empire ruled vast regions during Achuta Devaraya regime.

1. When Rani Mangammal’s husband Chokkanatha Naicker died in 1682, he was succeeded by her son Ranga Krishna Muththu Veerappa Naicker. Upon the death of her son in 1689, her son's widow was pregnant. Her son was succeeded by her grandson in 1689. Her daughter-in-law committed Sati, and Rani Mangammal became Regent Queen during the minority of her grandson. She was a great ruler of Madurai Naicker Kingdom.

Three different options followed when the king died without any heir during medieval times.