I don't know what they intend to make it, but it should be Numidian cavlalry. This was the most effective arm of Hannibal's army, and its superiority in battle enabled Hannibal to overcome his infantry inferiority (e.g. Cannae).
The other possibility is some kind of fast warship, like the quinquereme which gave the Carthaginians naval superiority in the !st Punic War until the Corvus came about.
Something which would suit Carthage's history would be an ability to hire mercenaries. The citizens of Carthage did not take part in foreign combat after the disasters they encountered against the Sicilian Greeks years before the Punic Wars. This was done by Carthage's huge mercenary army, hired in N.Africa and Spain. Their were no Carthaginian foot-soldiers the army of Hannibal that crossed the Alps (the one that everyone remembers).
Mind you, fireaxis will probably cave in to the popular imagination and make the UU an elephant.
The Celtic UU should be the Highlander, which during it's time was the world's most effective pound and charge unit. But since the Celts might not be there in this form, i.e. we'll have the Gauls instead, the UU might be Gallic Cavalry (they're infantry was comparatively ineffective). The Romans used these for centuries as their main cavalry. They were fast and powerful, the only thing that could stand up to the Parthians.