WI the Iberian peninsula remained Muslim?
The Caliphate of Córdoba remains united/is reunited at some point or a new muslim state emerges on its ruins. The northern christian kingdoms are either conquered already in the 8th century, or fall later on in a 'Conquista'. By the beginning of the 16th century Iberia is united under islamic rule either as an independent state or as part of a greater empire in the Maghreb. The peninsula would either be arabized or a romance language would emerge as dominant.
What then?
Would this state act as a western counterpart of the Ottoman Empire?
How would the absence of a Habsburg Spain affect the European balance of powers? Without a Habsburg block to counterbalance it, France might emerge as a dominant power of the continent earlier than mid 17th century.
Would the Netherlands fight its war of independence against ex. an Austrian catholic monarchy?
What if the extraordinary discoveries and conquests of 16th century Spain and Portugal in America were made under a muslim banner? What about a 'Semitic America'? (yeah, it's far-fetched, but still)
And finally how would the presence of a large muslim nation in Western Europe affect the cultural identity of a modern, 21st century Europe?