What if the Hafsid Dynasty (13th-16th centuries) of Tunisia had succeeded in capturing the island of Malta in 1429?
Can they embark in further conquests across the mediterranean?
How much wealth can Malta provide to the Hafsids?
Continued conquests through the Med. ? Unlikely; Tunis doesn't have a large enough manpower base to really threaten anything on the mainland. What it DOES do, however, is further strain relations between Aragon and Tunis, as well as shift the dynamics of Barbary Coast raiding. Southern Sicily is now much more exposed to North African piracy, and Christian Freebooters have lost a key base for count-raids, meaning I can imagine the wealth of Sicily somewhat flowing towards the east side and the Strait of Messina... with a corresponding increase in wealth to whoever holds the Straits.
The biggest impact, however, is if the Hafsids hold it long enough for Tunisia to fall under Ottoman control. In that case, not only is that strategic and easily-defendable island base in the Central Med. now available as a base for the highly dynamics Ottoman navy and that of their North African vassals (Who COULD seriously threaten Sicily), but the Spainish monarch is going to have to either let the Knights of St. John go their own way or offer them a different, less strategic and defensible base of operations. That could easily lead to the practical extinction of the Order.