My problem with the Huns isn't because they were short lived. My problem is they really don't fit the mold of a traditional civilization. I guess neither does Scythia, but at least they kind of made a city-list to go off of.
If the Huns do come back, they should act as a city-state and not being able to build settlers after founding their first "city". All other cities need to be taken by conquest.
What would their city-state even
be? Attila's Court?

Huns are a prime candidate for an Amplitude-style minor civ, though, which is something I hope we see in Civ7.
In Civ1, 2, and 3, Attila is the name of the default, "leader," of the, "Barbarians," who in Civ 1, 2, and 3 are an AI-only, "grab-bag," pseudo-civ with very limited potentials, cannot research or capture techs, inability to build cities, only capture them, inability to build land-improvement, only pillage it, cannot build improvements or non-military units in their restricted slots in cities, Government fixed at Despotism (the most basic, and default, type), do not benefit from the effects of Wonders in cities they capture (but the Wonders can be captured by a human or standard AI player), restricted to certain unit slots, that, "spawn," sometimes pop out of, "goody huts," and ambush, and other such behaviour.