Going to contribute to the outstanding civ needs:
I. Brazil
The stuff they have already points them the most toward a cultural victory, so it makes sense to align everything unique to them toward it.
Brazilwood Camp culture could do without the Acoustics requirement - maybe Drama and Poetry or Iron Working, since its the same one that gives lumbermill? Acoustics could improve the culture again, maybe break it down where D&P/IW start with +1 and Acoustics get +2?
Their unit comes way too late, which is a barrier for them. It makes sense on the one hand, because Brazil only came to be in the 19th century, but gameplay > realism.
The Portugese colonized Brazil in the 16th century, and were apparently in a constant conflict with the indigenous people, so something like a revolutionary would fit in with them. Maybe a unique Arquebusier? I don't know the language so someone else can think of the name.
II. Songhai
So I was looking at them for a bit and thought of something - nobody really has a wealth based conqueror theme in Civ 5. Songhai could fit that role because they built their Empire on the foundation of the Mali. You could say they can be envisioned as both, since the Songhai didn't last very long, while the Mali did.
Mud Pyramid Mosque could provide gold instead of culture so that religion is very profitable for them. This makes sense historically because they built their Empire on the former Mali empire, which was quite wealthy.
The gold bonus would let the Songhai be able to fund a larger military, making them a dangerous enemy. Of course, happiness would still have to budgeted so it wouldn't be overpowered since they don't get any happiness bonuses.
III. Siam
Gold is something that Siam could use more of, since Diplomacy requires bribing City-States. The Wat doesn't help with that. Siam needs either Gold or Influence from the Wat to compete with / beat its opponents for CS.
Gold would be easier to give, so maybe a % increase to the city for the Wat, or a gold bonus that equals the maintenance cost?
IV. Indonesia
Idk what Firaxis was thinking with this civ. Indonesia needs a victory type that their strategy helps them toward, and right now they have literally nothing that moves them toward a Domination, Cultural, Scientific or Diplomatic victory LOL.
I think a Diplomatic theme would make sense because there are so few of them and Indonesia is an alliance of many different cultures. Greece, Germany, and Siam could use a competitor. Maybe instead of their UA they could get Influence bonuses for gifting CS that they have trade routes with? Or maybe a resting Influence level of 20 that stacks with other bonuses? They could keep the UA they already have because frankly its so bad that you can add to it and it will still be iffy. They do have the best setup to mesh with Exploration right now though, as late is comes /shrug.
The sword unit could just keep Sneak Attack and dump the rest. The RNG is goofy and some of the promotions are just baffling in their worthlessness. A great flanking infantry would make Indonesian warfare devastating in the early-mid game.
Candi could make Great Merchants come out slightly faster along with the normal boost so that Indonesia can potentially dominate diplomatically with the massive boosts from GM every so often.