If after the Mahabharata war, Arjuna's son Abhimanyu and Draupadi's five sons would have remained alive, then who would have been the king of Indraprastha and Hastinapur?

Prativindhya, oldest son of Yudhisthira, was the heir to the Realms of the Kurus- and thus, by extension, all Bharatavarsha- until his death. However, because of his young age- still in the early 20s when he was murdered, it seems there had never been a formal announcement on who would've succeeded Yudhisthira.

Prativindhya left no sons in the original Sanskrit versions (so kindly desist from telling me about random beliefs from the middle of nowhere-nagar) and so the Crown ended up passing to the eldest surviving male heir in the line, Parikshit.