OP, you're basically asking for people to get their crystal balls out.

I'm with Esmjb on this: I think Brazil had what it takes to become THE epicenter of Covid-19. Whether things will have calmed down by NYE is entirely dependent on vaccines, herd immunity, who knows? Keep your plans loose: I'm fairly certain that even if things are somewhat back to normal by then, you'd be easily able to find last minute flights and definitely rock-bottom priced accommodation (especially with 1USD = 6 BRL, maybe more by then).