In the US, the Democrats and Republicans are already engaged in a primary process that will result in a presidential candidate. By the end of it, Republicans and Democrats will have their candidates for whom folks vote. It is, in a sense, the same as the UK in that the result is a "party leader," but a little different in that you're voting directly for this candidate.
In the UK, though, they also have a shadow cabinet. Right now the Labour Party is out of office but they have a Secretary of Foreign Affairs, Transportation, Education, etc. Or more precisely, they have a shadow secretary of Transportation, etc. Barring last minute changes, when voters bring in the Labour Party, they would not only be clear about who will be prime minister but who will be in each of the cabinet positions. The shadow cabinet members are known and they could be interviewed about policy, etc.
It would be fascinating for US candidates to name their shadow cabinet. It could tell us a great deal about their policy plans. At a minimum, one interesting question in a debate would be, "Which of your opponents would you most want in your cabinet? And in what position?"
I would give bonus points to any of the GOP candidates who answered, "Donald Trump. In a fetal position, still shell shocked about the way the country first seemed to love him and then turned on him like hungry wolves." But I digress.