Winning the Cisplatine War is the most likely PoD but carrying on keeping Uruguay will be very difficult.
Argentina's irredentist claims will still be there and they'll be waiting for another opportunity to have it "back". The Farroupilha revolution (or any equivalent) will be a great opportunity.
So, WI Argentina - a different one - has no irredentist claims to the area?
My thought comes to the
Republic of Entre Ríos. It was not really a seperatist polity but it could have evolved that way. It'd make a great buffer state! The Brazilians upon knowing of its creation could very well try to buy out its caudillo and both encouraging and enabling him to setting up a truly independent state there.
TTL Argentina will probably move its capital to Cordoba (like it has been suggested on Gonzaga's thread) and will be known by one of its original alternate names: United Provinces of South America. It may still claim to be
the successor state the Viceroyalty of the Río de la Plata but it'll know it is just one among others.
Having an independent Republic of Entre Ríos will be very interesting if Solano López isn't butterflied away. If he annexes Entre Ríos he'll fulfill his wish of a Greater Paraguay with access to the sea. He would have his focus shifted away from Brazil and Argentina and towards absorbing Entre Ríos.