to me that implies that the person/people in question are leaders of the entire world, whereas I meant more like individual heads of state.
Yeah, and in addition to that
dux is a very general term for "leader", whereas if you want to relate the modern sense of "heads of state", you might have to get a bit creative; no single existing term seems able to convey that meaning. I think of a modern head of state more in terms of being an administrative and political leader, a chief executive, than a "ruler" per se. With this in mind, you might have
praefecti politiarum ("those who have been put in charge of the states")? Alternatively, you might have
principes politiarum (saeculores). If you don't mind a Late Latin term, you might have
officiarii principales politiarum.
As an analogy to the foergoing, note that one of the official Latin titles of the British monarch is
Consortionis Populorum Princeps, which is officially translated as "Head of the Commonwealth", more literally "Head of the Community of Peoples".