What if England concentrated its colonies further south, closer to Mexico. Say that through a series of different decisions, Virginia becomes the furthest north of the English colonies, and other colonies are founded in OTL Carolinas, Georgia, west Florida, and Louisiana (beating the French to the last two). Lands further north are left to the French and Dutch. Florida is taken in the war of the Spanish Succession. By the mid 18th century British colonists are moving into OTL Texas and clashing with the Spanish outposts in that area. With the whole axis of colonization much further south, Spanish Mexico would replace French Canada as the main obstacle to British colonial expansion in North America. It would be a lot harder to capture than Canada, though - much more heavily populated, heavily garrisoned, and more important to Spain than Canada was to France.
I think I can come up with a POD for that one. The religious persecuted who formed the backbone of English settlement in New England was precipitated by the Pilgrims. They only ended up in Massachusetts because of storm. If the storm had forced them south, not north, then the axis upon which English settlement in America swings will be around the South. Britain's primary rival in the area from th 17th century onwards will always be Spain, not the Dutch or French.
For what I'm trying to do I'll need the American colonies pretty much as is. Maybe a stronger British presence in British Honduras and the Caribbean, and a denser population in the American South?
If all else fails, I can connect an Anglo-Spanish War with an earlier "Conspiracy of the Machetes" type thing. The English agree to prop up Criollos and give them a similar status to that enjoyed by American colonists.
Here's what I'm thinking, and tell me what you think:
Yellow fever plays less of a role in the War of Jenkins Ear.
In 1740, during the War of Jenkin's Ear Commodore George Anson's Pacific fleet is a great success and disrupts Spanish possessions in the Spanish East Indies as well as capturing a gold galleon earlier. Disease is not a major factor and most of the fleet survives the campaign. Meanwhile, General Oglethorpe manages to take St. Augustine and leads a mixed force into Spanish Florida, which remains starved by British blockade.
The following year, The British take Spanish New Granada in an earlier and more successful Battle of Cartagena de Indias then moving on to successfully attack and establish a presence in Cuba.
Mexican revolutionaries (Criollos and Mestizos) become emboldened by the failure of the Spanish to retain her colonies and make contact with Walpole to request recognition of and assistance in their rebellion. Britain agrees to support the rebels in exchange for them recognizing Britain's sovereignty over Mexico as a free Dominion. The compromise is accepted and Britain begins to focus its war against Spain into a war to gain Mexico.
After several years of back and forth, the Spanish cannot hold Mexico or the Caribbean. The Dominion of Mexico is established and the British use it as a penal colony for Irish dissidents.
I'm going to stop and get your feedback before continuing.