Spain remains a Great Power. Britain and the United States (among others) lose the Latin American market This would probably need a POD during the Napoleonic Wars.
It would be interesting how the US would try to get California in such a scenario.. war with New Spain = war with the Spanish Commonwealth.
Yes, I think the most obvious POD should be during the napoleonic invasion. Introduce not so difficult changes in the constitutive debate and the Constitution of 1812 and you have your fancy confederal
Sistema Imperial, as probably it could have been called (as in OTL). Also, with more widespreaded interpretations of certain articles of the Constitution as it was redacted in OTL, I think it could be likewise possible. The following challenge would be to keep the Constitution of 1812 in force enough time to consolidate the new system, though. Another option is Aranda's plan, or the similar Abalos' plan some years before or even another similar project concieved by Godoy latter. Probably it would have been more easy to make it stable and durable, but meh, it is a lot less epic than the 1812 option.
Regarding the markets, probably the freedom of commerce would be a precondition impossed by the american spaniards to accept the deal. If we use the 1810-12 POD, it wouldn't be probable but sure. That means that americans and british can penetrate the hispano-american markets, though on the other hand Spain wouldn't lose most of her market from night to day and not being so dependent on the british commerce the hispano-americans could enact more protectionist policies if necessary.
I foresee more stability than in OTL in both sides of the Atlantic, though not a haven at all. Possibly the fiscal history of the hispanic world would be different, and better, as well the lack of the legitimacy crisis created by the independencies would help. However potential enemies would be still a serious threat even with all the hispanic empire teaming up, not by chance the USA was a concern for spanish officials almost since her independence. The republican and independist sentiments are still around there in the americas, and in the case of the 1812 POD, Traditionalism could play also destabilizing a role, in Aranda's POD, it would be Liberalism. Beyond that, I have not enough imagination/knowledge to imagine a coherent developmento of the events.
As a final note, this scenario, assuming the system survives long enough, could have also interesting butterflies in the conformation of identities around the empire.
Cheers.