Following this line of thinking Dacia could be an important player as the Danubian plains are extremely fertile and the only thing that prevented these provinces (under the Romans) from becoming very prosperous were the constant Barbaric invasions.I tried to quickly find a map online showing soil types in Europe and the Middle East and failed. However, if you are willing to take my word for it, continental Europe had much more agricultural potential than either the Middle East or the Mediterranean (Greece/ southern Italy/ Spain/ North Africa). Continental Europe had heavier soils that required some additional tech to exploit. But of the areas in the Mediterranean and Middle East that had developed city based civilizations as of the time of Alexander the Great, only Egypt due to the Nile could match the agricultural potential of continental Europe. And before the industrial revolution, agricultural potential was decisive.
You should remember that Rome was an extremely militaristic nation and therefore there weren't many countries who could make a similar expansion.So whoever gets to continental Europe first wins, and that could even be the Po Valley, which the Romans conquered between the first and second Punic Wars. Better agricultural potential means more manpower. That means either some civilization develops in northwestern or central Europe (but remember the heavy soils), or a Med based civilization gets there, probably from Italy though possibly from Spain.
Po valley is unlikely to prosper as there was constant infighting between the Etrurian cities and the Gaulic tribes aren't the kind to create an empire.
As for Gaul as I've said in my previous post there would be many small kingdoms in the area as Caesar had noted that they were starting to create civilizations so 1st/2nd century AD there would be many small kingdoms who fight between themselves therefore a Gaulic superpower is very unlikely to rise from this region.