First, a disclaimer: "ethnicity" in the ancient world is a thorny subject. Entire books have been written on it. Even in Rome's earliest days, there were a variety of "ethnicities" in the city. The Caelius family was known to have a lot of red hair and fair skin (clearly a Celtic contribution). Others were olive-skinned and dark-eyed, like modern Mediterranean peoples. I'm not even going to touch the topic of ancient ethnicity -- I'll leave that to somebody else. I can, however, take some poetic license and muse about the origins of Maximus Decimus Meridius.

Trujillo (Latin Turgalium) was an outlying suburb of the enormous Roman city of Emerita Augusta (modern Merida). It was a colonia, the highest status of Roman city abroad, and was probably founded on top of an indigenous Celto-Lusitania site.(see note below). Augustus put the colony there in 25 BCE and filled it with thousands of discharged Roman veterans of the civil wars. Eventually, the cadastration (dividing of the land into parcels for veteran ownership) stretched all the way to Turgalium, but probably not until later, when the city became very large. It would eventually have all the trappings of a major Roman provincial capital: forum, amphitheater, circus, large temples, baths, multiple aqueducts, etc. (It's also an awesome and gigantic archaeological site now under UNESCO protection).

Given that, there are branching paths of possibility for the origin of Maximus.

Option one: he is a descendant of one of those veterans. His name, which is otherwise nonsense, seems to hint at that: Maximus Decimus Meridius (with the cognomen clumsily pointing to Emerita Augustua). In this case, we have some explaining to do. The vast majority of the settled veterans would have been centurion or lower, and there was a very strict glass ceiling above this rank. Our Maximus is a legatus of Marcus Aurelius, and that is an elite, privileged station, almost certainly Senatorial. Would it be possible for the descendant of a regular soldier, generations later, to achieve this? Yes, but very unlikely. Perhaps our original colonist ancestor works hard to establish himself as a leading man of the new colony. His competition will be the local elite, who have an incredible advantage on him as far as financial resources and also have a driving ambition to achieve in the new Roman political system. They will want to be aediles and duoviri of the new colony, to integrate themselves into the civic structure, and from there to reach towards political achievement at Rome. Septimius Severus, for instance, was the first "African" emperor (from Tripolitania). Our settled veteran would have an uphill battle and would need to be savvy himself, and have savvy descendants, in order to propel Maximus so far in such a short amount of time. So for option one, I think it is unlikely. As far as what he looks/sounds like: in this scenario, he probably speaks something very close to unmarked Latin with a slight "provincial" accent, possibly identifiable as Hispanian but possibly not. He might also speak a little Greek, and probably poorly (an Athenian would sneer at him like a Parisian sneers at me in the present day). He is several generations removed from that original veteran colonist in 25 BCE, and some native blood has probably entered the family through marriage of local women, but linguistic impact is probably negligible. That initial veteran was quite possibly a bumpkin from some rural Italian town, but the army speaks Latin and colony life is a lot like the army, with more luxuries. Over generations, this has a leveling effect. He is most likely olive-skinned, dark-haired, and brown-eyed, as were most of the peoples of Italy in the first century.

Option 2: he is the descendant of an elite "indigenous" inhabitant of pre-Roman Emerita Augusta. His ancestors were probably already Romanizing before the colonia was founded and hit the ground running as Hispano-Roman local elites in 25 BCE. They were early aediles and duoviri of the town, paid out a lot of money to (help to) build city projects like aqueducts, roads, public spaces, etc, and as magistrates of a colonia received Roman citizenship for themselves and their descendants. They probably already knew Greek and would quickly learn Latin. They would regularize their name with the three-name Roman system (which often produces "weird" names by Italian standards, so that's a plus). By the time Maximus comes, a few generations later, the family is as Roman as you can be, reading Vergil in a Roman-style villa or domus with an impluvium and a red-tile roof, speaking Latin (and Greek), sending kids to Rome or Athens to study, thoroughly enjoying the chariot races at the circus and the animal hunts at the arena, enthusiastically participating in the many Roman religious festivals. They drink Roman wine, and they produce fish sauce to export all over the Med. A few family members are or have been the flamen Augustalis, priest of the divine Augustus, further enhancing their prestige. The name of the ... gens Decima ... is plastered up on walls and carved into marble all over Emerita, which is becoming quite large and important. Maximus is sent to Rome to study under an important official, perhaps an advocate or someone close to the Imperial apparatus. He makes friends in the right places and is ushered into standing for and winning a quaestorship. He goes off to a province with a proconsul or prefect and makes more friends, as well as further enriching himself. Back home, he orders the construction of a temple in one of the Emerita suburbs. By becoming quaestor, he has earned a place in the Roman senate. He lives in Rome and participates further in Roman politics, moving up the "path of honors" to aedile, praetor, and then consul. Marcus Aurelius likes this man from Emeritus -- he has a good head for military matters and has good experience in the field. The Emperor chooses him to be one of his lieutenants (legati) in the war with the Germans, and here we are at the beginning of the movie. He speaks impeccable Latin (and if he didn't when he arrived in Rome, it was quickly corrected). Like his alternate-universe colonial-descendant option, he is probably olive-skinned with dark hair and eyes, but it is very difficult to predict. Were his pre-Roman Lusitano-Celtic ancestors fair skinned and red-haired? If so, were those traits preserved? Hard to say.

(note) As an aside, the Lusitanians were distinct in Iron Age Iberia in relation to their Celtic neighbors. Their language was different and their culture differed in some ways. I think there has been a lot of argument about whether they are Celts or not, but I'm not an expert on that).