Well, there could be an intermediary between the kings and the imperator, some sort of imperial administrator if you will, that would govern & command in the name of the emperor. But it would only be a utility rank. There is no actual gap to be filled by a middle man between a King and an Emperor if the empire is an aggregation of kingdoms.
You could however decide that the empire is split into provinces that each gather many kingdoms, which would make the governor of said province above the kings of each kingdom it englobes, and below the emperor which federates the whole.
Imperial Administrator is a good rank. Although, I suppose you could have a Council of Kings, which, as a body, is not a challenge to the Imperator but is a kind of check against the Imperator's power. The group is powerful but only if working in perfect unanimity.
If you had regional collections of kingdoms, as mentioned above, High King/Queen could work. But otherwise there isn't really anything that fits as far as actual governing.
One tip: we moderns are obsessed with hierarchy in a way pre-modern societies simply weren't. The Romans, as well as medieval Imperial administration, were not at all bothered with duplication of authority, giving a title to one with one set of duties and the same title to another with different duties, and so on. Moreover, titles change over time in theoretical meaning and even more in practical power. Titles tended to mean exactly what the people involved wanted them to mean.
In the Roman Empire there was a period where there were two "levels" of emperors but I think simply borrowing their titles will confuse matters for you and your readers.