Lol! settle down Beavis! Trump isn't "king" of shit. There will be no unelected monarchy in the US. He's going to "murder his opponents"? Yeah, let's see if he gets away with something like that. Trump isn't running in 2028. He might not even be alive by then. And if he is, his cognitive decline will be more than it is now.
He may try to promote Don jr to run after him. But in reality, Trump doesn’t care about his children, apart from Ivanka and then only in a creepy way. He’s ironically too selfish and narcissistic to create a dynasty; his children are not him, and therefore are unimportant. His relationship with them, like his relationship with everyone else, is purely transactional. What he wants is to be adored while alive and after he dies. If his kids end up drunken failures, that’ll just highlight his awesomeness even further. Planning a hereditary dynasty requires rational acknowledgement of one’s mortality, since it’s fundamentally about guiding what happens when one is gone, and Trump is so pathologically self-centered that he can’t imagine a world without himself in it.
Trump himself may not be planning this, for all the reasons discussed above, but I have little doubt his spawn are thinking about how to do it. They have never accomplished anything that didn’t involve riding daddy’s coattails, so this will just be more of the same.
Now, his cult may see the Trump name as the ticket to undying power, and may be making plans to elevate the kids and ride them as far as they’ll go. But cults of personality have a way of evaporating when the subject is gone, so I’d expect a self-destructive knife fight on the Right within a couple of years of the Trumps demise.