Answering the question of the op, I would say that Portuguese expansion was extremely likely or even inevitable for it to happen for a variety of reasons.
For example, the colonial Portuguese government in the 16th and 17th century many times officially paid or at least gave support to Bandeirantes (the explorers) who were ordered to look for gold and precious gems (the main impetus) but more often than not they did something much more important: they created trails when exploring rivers, jungles and rock formations, meaning that even if the only thing they returned with was a couple of enslaved natives, they had also created maps regarding the places and people they found, which way certain rivers flow, the best way to travel through a mountain pass, etc.
Now this is extremely important because it gave the Portuguese authorities a idea to where they should better settle their cities and where to build, meaning they knew where it was a good place to build in order to build a bigger presence(see Belém and how for many years it was basically the main influence in the Amazon river and able to rule all over the lightly populated Grão Para)
One not must discount the ox too, for much of the 17th to early and mid 18th century, the settlers fought the "Guerras Bárbaras" (lit Barbaric Wars) where because the raising of cattle and it's expansion in the interior was growing very rapidly, it led to disputes with the natives living in the interior and eventually their extermination, ensuring that the countryside was all but controlled even if not fully populated.
The end result is that unlike Spanish America, Portuguese Brazil was much better in feeling unified and relatively easy to communicate with because of the way it expanded, giving it a edge of needed to grow more that other colonies did not have (with some exceptions of course)
This would continue through the empire and republic, as people often forget that free navigation through the Plata River was important for Brazil to keep better contact with the Mato Grosso region and Solano Lopez attempting to better control it was one of the many reasons for war, because when you have a country big like Brazil communication is always important.
As a result, if we're talking about potential Brazilian expansionism all the way back towards Tordesilhas, we would see Brazil retain the Colonia of Sacramento(in the most likely case winning the Guaranitica War to solidify their claim) meaning they already would have a enclave in Uruguay they can use to expand on, a Argentina might not form if Portugal is there to take control of Buenos Aires or at least the surrounding regions to protect it's Plata River navigation as well as taking areas in Bolivia and Paraguay to better connect certain regions or "get natural borders", french Guyana is a question if Portugal can retain it during the peace of Vienna, which is hard considering it was a trading peg for them being able to continue the slave trade as well as having their claims on Olivença (a long disputed region with Spain) confirmed, so somehow strengthening their hand will help them keep it.