If Wanking means territorial gain, ain't gonna happen. However, you can make Argentina and Brazil into major powers, beyond OTL. Takes a bit of magic, but there's no reason 2 big powers can't develop along side each other. get rid of the War of Triple alliance, and you can allow Paraguay to grow into more of a power. Dreams of expanding to Bolivia/Peru are fantasy. Paraguay got to be Paraguay precisely because it was so isolated from everything else. You might as well wank Brazil taking over Venezuela. P has one direction to go, and that's to Argentina and Brazil, and your challenge automatically precludes that.
fantasy wank could possibly have Argentina retain more of the plata viceroyalty. Paraguay could take more of Brazil (especially if the crown doesn't transfer there). Brazil has nowhere to go territorially except to hold onto Uruguay (assuming the original challenge of A and P being wanks, too). Expansion through the amazon is ASB. Although, you could wank Brazil by keeping the crown there and taking colonial possessions from Portugal, namely, Angola/Mozambique (and everything in between).
So: A keeps viceroyalty. P takes corrientes/missiones (maybe Entre Rios) P can expand a bit in a few directions - nothing of importance, but the map gets bigger. Brazil gets colonies. There you go, a three way wank.