The sun rarely sets on the Portuguese Empire!
The past twenty years have seen a switch in my primary ally from Aragon to France. In the early 1530s, Aragon started a war against Castile and called me in, despite all of my troops being in Asia. Unsurprisingly, they were not able to get back in time to be very effective. Mercenaries did manage to inflict some losses on Castile (at the cost of considerable debt), but Aragon ceded Aragón just as my troops had made it back from Asia, ending any chance of turning the tide. When Aragon started another war at nearly as foolhardy a time six years later, the alliance was over. (This is a 1.6.x game, currently 1.6.2, so Aragon wasn't breaking a truce)
But in 1547, both France and I were ready for some military action, particularly as France had the world's largest army by a factor of two and couldn't start any wars due to a regency. So I declared war on Castile, and proceded to trounce them with a healthy dose of French military victories. In the end I got four provinces in Iberia and all of Castile's remaining colonies (having already annexed two small colonies outright). The 158.2 overextension (of which 126.2 came from this war) is probably going to be somewhat problematic, but it feels good to have no Castillian colonies in 1552.
All the while, I've been colonizing with a focus on estuaries and centers of trade. The St. Lawrence, Mississippi, and Parana estuaries are Portuguese in the New World, while Cape Town is Portuguese in Africa along with the Niger Estuary (a recent conquest), and a thus-far-totally-useless center of trade in India (which is too far away to core or send a merchant to). All three estuaries in the Sevilla node are now Portuguese as well, giving trade dominance in our home node. The Hudson River estuary is Portuguese as well.
I haven't figured out why I can't make Castile a rival. I would have thought they were more powerful than me before the war, and they're definitely less powerful now, yet they haven't been an option either time. Along with Vijayanagar and Mali, my current rivals, my only other choices are Lenape and Qara Koyunlu. The latter really doesn't make sense as I have no borders anywhere near them. Maybe Castile will become eligible after they rebuild a bit - currently I have infinitely more powerful armies and navies, so perhaps the war took them all the way from "too powerful" to "too weak" at once.
Edit: Found out why Qara Koyunlu was a possible rival - they've colonized the Maldives!
Apparently they chose the expansion idea track, got a port on the Persian Gulf, and decided the Maldives would be a splendid location to expand to. The Maldives share a sea border with my Indian province, Kochin, so they're a valid rival!