Would slavery in Brazil have ended earlier or later if it had annexed Uruguay during the Cisplatine War?
Brazil's coffee and sugar industries were largely enabled by huge amounts of slave labour, and following the abolition of slavery in 1888, plantation owners lost a significant amount of their sway over the nation, and thus began "milk coffee politics", wherein dairy and coffee oligarchies -- less reliant on enslaved labour than the sugar industry -- dominated the country's local governance.
My answer to my own question is that Uruguay (aka Cisplatina, which historically had a small slave population, would have made it so that Brazil's economy was proportionally more reliant on free dairy farmers, and less reliant on subtropical sugar and coffee production, thus hastening the disenfranchisement of slaveholding plantation owners. It probably wouldn't have had a profound effect on the abolition of slavery, but I imagine that it wouldn't have been nothing.
The best counterpoint that I could raise is that landowners in Uruguay would have taken advantage of the availability of slave labour in Brazil, compared to Uruguay (or rather, the region which is now recognised as Uruguay) which was already in the process of gradual abolition prior to the Cisplatine War. Hence, the territory would not have remained low on slaves; eventually reaching a similar slave-per-head ratio to Minas Gerais, Brazil's largest dairy-producing state historically.