Another Italian city state we hardly think of, or a Kingdom like the Etruscans?
Well, Etruscans may have a far better rap, would it be only because they influenced
much continental Europe, approximately as much as Greeks in the West, maybe more in central Europe.
Now would their decentralized occupation of Italy, with their league being more of a ceremonial institution than a real and coercive one, be able to really repeal Celts and take over Italian federations (as Samnits)? That's another thing.
I would rather see them having the same role than Greeks in Balkans : keeping their independence, influencing much their neighbours, and forming some hegemon of their own (maybe imperial ones in the long terms)
Basically what slydessertfox said with some reserves (see after). Carthage was quite similar to Rome on many regards, but may have lacked the territorial anchorage that helped them to eat little by little their neighbour up to be not only hegemonic but monopolizing politically.
In spite of a strong authority, Carthage never really managed to fully unify or force to unify familial interests or other submitted cities interests.
A Macedonian or Diadochi based empire?
A bit too distant and focused on the wealthier East (with as well more dangerous rivals). If Seleucids manage to get their shit together, they could form an Helleno-Persian entity that may become a superpower with hegemony in Eastern Mediterranea (think Ottoman Empire lines), but I doubt they would focus on the western part (less rich, and risk of being overstretched)
An enduring Persian empire
Well, it does have more chances to end as a superpower (see Seleucids), but it would be probably more focused, as IOTL, on Eastern Mediterranean and Arabia than interested going west, when using client states/vassals or further "friends".
Definitely not. Celts is essentially defining a civilisation, an ensemble of cultures with common references and ground but extremly diverse politically.
You could most probably see the rise of Celtic kingdoms/hegeoms/confederations (more or less hellenized for some) as IOTL and an enduring strong celtic power in North-Western Europe, of course.
a Berber or Mauretanian empire
Maur and Berber or Numids designate roughly the same group of peoples. Basically...with Carthage still on the ranks and not annoyed by Rome, all of these are likely to remain under Punic "benevolent supervision".
a Greek-speaking polis in Greece, Sicily or the west med pulls a Rome?
Well, you have room for local hegemony. Sicilians and Gallic hellenistic entities proven able IOTL to establish their dominion on their neighbours relativly easily.
It's to be noted they did so with the indirect or direct roman support, though. I'm not too sure how well they'll turn with Carthage there.
an empire based on some now obscure ethnic group like Thracians, Illyrians, Dacians, Phrygians or other Anatolian peoples?
Well, Dacians proved being able to pull an imposing state (compared to previous situations, with diverse tribal states and confederations) under Burebista, and I don't see why it shouldn't happen anew.
That said, Illyrians were extremly diverse (again, a convenient name for people as different as Iapydes (mix of Veneti and Celts) and Dardani (mix of Illyrians and Thracians)
Pre-Roman Spain is a
clusterfuck of peoples with diverse and non-exclusive influences and identities.
There again, you could possibly have local hegemons (as with Elesyces), but Punic influence (commercial and political) would be still quite huge.
You'd tell me that pre-Roman Italy wasn't really better. Fair enough. But it was a region roughly unified culturally and politically by Etrusceans and hellenistic influence : Spain was less so and offered a less interesting position.
A Germanic, Scythian, Slavic or Sarmatian empire
Germans, as we understand it, would probably not exist. It was essentially coming from the differentiation between Galli and Germani made by Caesar on a geographical scale. Effectively, the difference was limited (originally German people were deeply influenced by Celts, if not Celts themselves)
Slavic peoples, for similar reasons, may end not existing at all, ethnologically speaking. The pre-Proto Slavic (meaning not even the group before Slavs) were generally included among Scythians or other great groups of the region and didn't formed distinct strong groups we know about before the Middle-Ages.
Scythians and Sarmatians are roughly the same thing. I could see them pulling an Avar or a Magyar in Europe, but I don't think they would have the structures or power to takeover the best parts of Mediterranean Europe. More likely form hellenized entities as Celts did before.
Maybe taking over Persia?
None really. The best I can see Carthage doing is getting control of Massalia over time on the coast of Gaul (without Rome to protect them Massalia is going to have to turn to some power for protection or get conquered)
Celto-Ligurians and Celto-Iberians would probably take the best of it if Massalia is to be conquered. Federations such as Salyes in Provence or Elisyces in Languedoc seem to have known quite an inner dynamism by the third century BC.
(IOTL this dynamism was broken by roman interventionism in the region. Without this, I could see it continuing mixing native, italic and hellenistic features)
Doesn't mean that these entities couldn't have ties with Carthago, but I'd tend to think that it would be more on clientelist and commercial influences lines.