I know that it was a hard nut to crack even otl. I was hoping a fictionalized Arabian superman at the head of some unstoppable horde would view Central Europe as a prize and overcome (or assimilate) the native opposition, gaining valuable manpower as well as invaluable lumber and mineral assets for his middle eastern capital...
Ideally the point is to Arabize everything from Germania to the Balkans while forcing the Romans to focus on Western Europe, Great Britain, and Ireland.
If Rome eventually counters and drive the Conqueror's territory back to middle Asia, at least I have a partially Arabized Central Europe that Rome now has to partially Romanize, as well as a fully Romanized Britannia and Hibernia.
This also makes the Arab world easier to unite, and the conquest of Parthia opens lots of doors to earlier useful contact between China and Europe/the Middle East.
As I said, it doesn't necessarily have to last to have pasting effects: Germanic tribes with darker skin and curved swords, the European camel, Roman Britain and Ireland, and direct contact between China and the west are enough for me.
Also, if close enough to the time of Jesus not to butterfly him (if you are of the school that believes he was butterflyable) the development of Christianity in both the Arabian Empire and ttl's Rome is fascinating. Lots of potential, I think.
Edit: An idea has come to me... What if after a partial or complete conquest of Arabia by a slightly longer lived Alexander the Great, he has an heir by an Arabian Bedouin chieftain's daughter? Could Alexander's Arab son or grandson achieve what I have envisioned?
Personally? Nah, he would do for the lands of the Roman Empire and nothing else. Ther is, literally, nothing in Germany worth conquering to the mind of a late-antiquities conquer; no cities, no great resources. Nothing. if this *Great Man explodes into the scene, he is going to take the East (hence why the Empire will be Hellenized quickly) and stumble into Italy. Maybe, if he's lucky, he will take Italy, but there is literal no need to take Germany of the Slab lands. There is too little to gain compared to too great an exertion of resources.