It did, it's called Argentina
If you're talking about an La Plata including Uruguay and Paraguay, the first one is 'easy': No Cisplatine War, or one won by La Plata. Another POD would be the adoption of a more federalist constitution earlier, thus preventing Uruguay from drifting away.
As for Paraguay, it would be more difficult but not implausible. Military conquest would be easier if the Paraguayan Expedition had funding. At all. There could be some diplomatic solution, but it would be very early in the revolution, since there were several political and even cultural differences... Frankly it's not my area of expertise, but I can find you something.
If you're looking for an *Argentina without civil wars... well it's a tall order, but there are lots of PODs to choose from... And lots of butterflies.