Ok, lets assume they attempt this, eventually much of christendom tries to stop them . In coalition with muslim iberia, they succeed, taking everything south of river po, while all of iberia south of pyrenees is muslim or muslim vassal.
I don't think such an alliance is very likely. Neither side liked the other all that much. I don't see the early Fatimids going with the late Umayyads, if only on religious grounds. Also, the Andalusis hold territory well into Switzerland, so they know intimately the logistical difficulties and questionable value of such conquests.
- Stronger Islam this timeline, and if these realms sustained for long enough, muslim rather then christian reformation.
If Italy stays Muslim (rather than just Muslim-dominated), this will shake up history to the point that the Reformation becomes a meaningless concept. But I suspect rather that it would end up like Iberia or Sicily, a Muslim-dominated realm with a mixed Muslim (immigrant and convert), Christian and Jewish population.
-Presmably Italy splinters in to muslim city states only formally tied to fatimids, and similar to otl italian city states-hubs of trade, culture, and incredibly powerful vis-a-vis the rest of the world for their size.
I doubt the second part. Italy's cities managed to leverage their role as Western Christendom's toehold in civilisation into economic dominance over the rest. Integrated into the Muslim world, they would turn from a developing entrepot to an undeveloped area into the underdeveloped periphery of a highly developed polity. Europeans would not come to Italy to study, Italians would head to Cairo, Baghdad and Kairouan instead.
-If italy isn't majority muslim, their would be a significant chunk of muslims(20%?). I'm thinking concentrated in major cities. Hence, even if reformation starts with christians in italy and iberia, could soon spread to muslims in those areas as well.
The first is likely, but there is little evidence that either Christians or Muslims in al-Andalus or Sicily took much interest in each other's religious affairs. If Christianity underwent Reformation (which, without a powerful papacy and its attendant abuses, is unlikely), the Muslim overlords wouldn't be likely to notice much beyond a bunch of dhimmis going nuts over some kind of doctrinal dispute. Best case scenario is they clamp down on the violence and force everyone to live and let live.
-the fatimids might take egypt eventually as well(they will be wealthy from taxation of italy), and be focused on east med and lose interest in italy.
Once thy hold Egypt, Italy will likely become a sideshow, if that. But Italy's taxation isn't likely to be enough to give them an edge over the opposition. Italy's naval resources, on the other hand, could do wonders for a Fatimid state bent on maritime dominance. Lack of suitable timber, iron, pitch, tar, hemp and flax often hampered North African naval powers, and Italy's sea cities initially made their money exporting these things.
-So these states may be all but independant, with the exception that the fatimids tax them. They will probably soon rebel as well, if they get powerful enough.
More likely just wait to let the whole thing collapse on itself. The problem is, any Muslim presence in Italy will need a strong protector, and the available protectors in the area aren't exactly what you'd call paragons of multiculturalism. I don't foresee anything like the Caliphate of Cordoba.
-Alternately, the fatimids might become itlay centric or be other thrown and replace with italian caliphate(caliphate of rome, anyone?)
In this case might take territories in east med just to secure trade routes.
Rome has no horse in the Caliphate stakes. If the Fatimids claim the Caliphate, it will be tied to their family background, not their conquest of the City. That's why I foresee trouble - holding Rome has no side benefit to Islam, but recovering it means the world to the Franks and Anglo-Saxons. And I can't see them conquering territory to secure their trade, unless they want to go maritime power. From the Muslim POV, trade *comes* from Egypt and Syria and *goes* to Italy and Sicily. You wouldn't send ships from Nigeria to secure territory on the US coast to 'secure' their oil trade, would you?
-Need anyone say it, no real crusades as well, with more christian influence on Islam, so less radical islam
The latter is likely in the short term (you have to live together with all those Christians who are used to running the place), but as I said, the Muslims of italy will need a strong protector, and most military powers out of North Africa were not the tolerant type. I don't see good prospects there. More likely a lengthy history of conflict which radicalises positions on both sides, with a regular influx of crusaders from the north and ghazis from Norh Africa.
