The first one is how Brazil keeps OTL Uruguay. Is Argentina involved in the war or not? How does affect Rivadavia's government? Is there the need for a unitarian uprising, or Buenos Aires goes to war with the provinces yet again? Butteflies, butterflies.
The other one is that the OTL War of the Triple Alliance is completly butterflied away.
Personally I think that Brazil had no conditions of keeping Uruguay after the Brazilian independence. Firstly due to the economical crisis created by the separation with Portugal (Brazil was completely broken during the 1820's). Also, and locally most importantly in the case of Cisplatina, was the division of the local Portuguese forces between those loyal to Lisbon and those loyal to Pedro I.
Until 1822 General Lecor, the governor of Cisplatina, had the support of the elite of Montevideo (and some other cities) basically because he managed to ensure a peaceful period there. Before that, from 1811 to 1817 the city was occupied by the Spanish, the Porteños, the Artiguistas and finally by the Portuguese. Lecor convinced the the elite of Montevideo that only the Portuguese forces could keep the peace. Also, he distributed condecorations and lands among the supporters of Portugal, and made his officers marry the daughters of important landowners (even he married one, Rosa de Herrera y Basavilbaso). When the Cisplatine Congress of 1821 happened, the delegates decided for the incorporation of the territory to the Portuguese monarchy as an authonomous state. It means that the Cisplatine would have an administration separated from Brazil, sending their own representatives to the Courts of Lisbon, but under the same king.
When the independence happened the garrison of Montevideo remained loyal to Lisbon, and Lecor, who supported Pedro I, needed to flee to Canelones. From there he organized a siege against the city, reconquering it later. However, those events showed to the people of Banda Oriental that the Portuguese were not so effective to keep the peace in the territory. Also, it caused the departure of the Portuguese troops back to Europe, leaving a smaller occupation army, and it started the reaction against them.
The obvious POD for a Brazilian Uruguay would be make the garrison of Montevideo support Pedro I instead of Lisbon. However, even if it happens Brazil would still deal with a financial crisis due to the independence that might ruin the Brazilian policy in the Plata. So other option would be to make John VI never going back to Lisbon, remaining in Brazil with the Portuguese Treasury. But that creates other problem, as John VI was actually contrary to the annexation of Banda Oriental. He wanted the territory given back to Spain, as a way to please Ferdinand VII, or at least independent as a buffer state between Brazil and Argentina.