This is probably just a thought but, as far as I have read in most threads it seems that:
* as long as most threads show that British Empire (or the USA) could have been bigger, more powerful, and IOTL they were as not powerful, I must conclude they had poor rulers and military leadership,
* as long as most threads show that the Spanish crown could have been smaller, much weaker, and IOTL they were more powerful than in any TL, I must conclude they had great brilliant rulers and extraordinary leadership,
So I propose the following POD, group of PODs:
* the Hapsburg do not inherit the Spanish crown but the English one, so we have the combined power of England, Burgundy and Austria,
* the alt-Trastamara in Spain keep a low profile as junior partners of the English-Burgundian crown, although they keep the burden of the American possessions (just a mean of keeping them weak).
How big could be that anglo-burgundian empire?