Just watched the trailer, and I have some theories on what this movie will ultimately be about.
Spoilers for Endgame and possible spoilers for Far From Home ahead:
It's clear to me that Far From Home is not only meant to be about Peter continuing to grow into his role as an independent superhero, but that this movie is also going to explore what that means for a world that no longer has Tony Stark.
This trailer seems to be laying out a lot of the road map for how this movie is going to go.
We start with Peter just after the events of Endgame, doing his best to keep New York safe, but he's struggling with the loss of his mentor and father figure, Tony. It's likely we'll be seeing the world's reaction to the death of Iron Man as well, and I have no doubt this movie also serves as a bit of a meta-commentary on that very same event.
Anyways, likely exhausted after the fight for the universe and likely feeling unworthy seeing Tony's face everywhere, Peter decides to take a break, catch up with everyone he hasn't seen in five years, and most importantly stop being Spider-Man, at least for a little while.
Now, we get to the villain of this story: Mysterio. Quentin Beck has traditionally been in the comics as a vengeful and spiteful movie effects engineer who uses his technological trickery to get what he wants. In his first appearance in the comics, he framed Spider-Man for various crimes and attempted to get rich and famous by posing as a new hero who came to take Spider-Man down. Given what we likely have in this film of the world looking for a new Iron Man, my theory for the main conflict of the movie is this:
Beck wants to capitalize on his mastery of technology and the world's loss of Iron Man, so he orchestrates a number of attacks using his special Elemental "creatures." He poses as a real superhero from an alternate reality caused by the Snap and attempts to set himself up as a new Iron Man.
Nick Fury (who I have a sneaking suspicion might know Beck is a fraud), calls in Peter, likely because he trusts him as Tony's successor. Through whatever means, Peter will eventually realize Beck is attempting to take over Iron Man's legacy, and Peter will have to find the courage to step up, stop Beck's plans, and take his place as an independent superhero and "the next Iron Man" in a figurative sense.
Really, it's looking to me like this movie might be fairly predictable, but by no means a bad film. It's the next step, it seems, into turning Spider-Man into his own thing, not only paying homage to Iron Man and his legacy, but passing the torch on to Peter.
For anyone who didn't care for "Iron Man Jr.", this is definitely looking like a step in the right direction for you. If you miss Tony, this might be a good send-off for his legacy and might help give some closure.
Overall looking forward to it, really excited particularly to see Mysterio in all his fishbowl glory on the big screen!