Given his selfishness, if he did name an heir, it would either be one of his kids (so he can continue to see his name on things) or it would be someone he thought he could exert complete control over and basically stay in power with them as a puppet.

(And for the latter purpose, someone obscure, currently barely known would be ideal, someone who owes everything to Trump and has no independent base of their own. But also needs to be somebody with deep financial ties to the Trump family and a lot of debts to be paid off. Oh, and preferably also involved in the whole Epstein situation, to help control them through blackmail.)