I'd suggest checking into the Gauls as a possible source for a Mediterranean-spanning Empire. Too often they're overlooked, and they have a lot of the advantages that Rome had - they were just slower in developing from a tribal society to an urban society. They were on the way to unification under the Arverni tribe (modern Auvergne in France) and to developing big cities like the Greeks and those other "civilized" cultures, but the Arverni hegemony was broken up by Rome in the late second century BC, and Gaul was of course conquered by Caesar soon after.
Hm, a Gallic power in Europe? I like that idea. But wouldn't it be more likely to become a continental as opposed to a Mediterranean power?