Well, if we assume that by some measure the Spainish manage to seize Mexica around the same time as they landed in the north IOTL, it would have netted them boatloads of gold during the critical years of the Turco-Hapsburg naval wars over control of North Africa and the central Medditeranian. If they had access to that capital (as opposed to the trickle of silver from the Peruvian Silver Train, which they either ended up dumping into China and having to ship practically around the world or lose 3/4 of it to pirates, ice flows, storms, and sinkings on the Terra del Diablo Fuego route), they might have been able to afford a fleet that could effectively contest that of the Ottoman Sultans and his Maghrebi clients, or support thePortuguese in their attempts to crack into the Swahili-Ottoman-Hindustani dominated Indian Ocean trade networks to the Spice Isles. That could have some real knock-on effects in terms of keeping the Medd as a viable outlet for Christian commerce, the economic survival of the Italian city-states, and the colonial fates of East Africa and East Asia. Would we see European empires in Swahililand, Somaliland,central Africa The Indian subcontinent, or the East Indies like they eventually created in South and West Africa and the Americas?