In Civ IV, the Zulu were the kings of ancient/classical-age harassment and pillage. Impi were second-to-none for going out and utterly demolishing even the best-developed enemy empires via massive pillaging and worker stealing, while outmaneuvering and using terrain defense to outfight (where necessary) enemy forces, and using their speed to lightning-strike weak new cities. You could easily pillage a whole continent back to the stone age, then come in with axes and swords and cats at your leisure to pick off what was left of your crippled opponents.
They were also great at expanding like crazy without crashing their economy (via ikhandas, cheap workers and cheap granaries), so you could have truly massive populations to whip-rush like crazy.
It was a really fun playstyle, and something I'd kind of like to see replicated.
Zulu were all about mobility and outflanking. While the impi is pretty much guaranteed to be a no-movement-penalty kind of guy, something that I think could be really interesting and powerful as a UA would be "no movement penalties for terrain or ZOC" for all units. And then make the Impi maybe a 3-move spearman with an increased flanking bonus or something.
Maybe too powerful? I dunno, though it would certainly require an altered playstyle to fully take advantage of, which seems to be really what they're going for with the DLC.