Vietnam was embargoed until the 90s, and dropping it basically allowed the soft power of the US to do it’s thing. 'Nam isn’t really an ally of the US, they consider themselves neutral, but they’re undeniably very friendly. I suspect that a generation of trade and tourism could do the same to our relationship with Cuba and might result in softening attitudes among Cuban-Americans as they reestablish contact with their families and reconcile lingering animosity from the revolution.
I think this would also work for the DPRK, Iran, and others. Trade is really nice and children are rarely willing to carry the grudges of their parents.