Is it possible to create a complete vacuum of power in the entire Middle East and the Mediterranean by destroying these empires? If this did happen and Islam had been previously butterflied could any new civilization have tried to dominate the region and where would that civilization come from?
Hm, you might try this scenario:
Roman Emperor
Aurelian was killed in the campaign against the Alamanni in 271 and they took Italy. The Goths invaded the Balkans.
The
Palmyrene Empire might become the strongest power in the region. It might take Mesopotamia from the Sassanid Persia or maybe some other regions. Septimius Odaenathus was surprisingly victorious against the Persians, if you leave him alive - he might even annihilate Persia or make it an insignificate state like they were during Parthian domination.
That would have been quite an alternative history in a quite alternative world...