Michael Keaton as the Vulture was surprisingly good. He was cold, menacing, has good motivations. I was surprised to see him as the father of Liz Allen... It worked very well, though, because...it gives him a great motivation with family that makes us truly understand what he's going through. His backstory with being screwed over by higher authorities and Stark, and his profession as a scavenger, helps very much as well.
Part of the reason while Keaton was the best thing in this movie while simultaneously also feeling wasted was because every encounter between him and Peter before Homecoming was them behind masks (they're just obstacles to each other, not people).
Here are a number of things I would have done differently:
1. Peter Parker. My take on the character for this film would be a sort of deconstrustion of the Webb Series Peter Parker, he's still kinda self-absorbed and arrogant, wondering why the world seems to treat him as it's chew toy (I actually like this more than I thought, since it would be a bigger, less obvious homage to Ferris Buller). He also kinda took the wrong lesson from Uncle Ben's death. To him, "great power/responsibility" is him trying to use his powers anyway he can and profit off of them somehow in a non-descript way. To Peter, joining the Avengers is the fastest way to setting him and May for life.
2. Ned Leeds. In addition to providing comic relief, he's also the guy who films youtube videos of Spider-Man (referencing how he's a photographer in the comics), and he monetizes those videos because he knows they'll make a lot, causing a 60/40 profit split favoring Peter (because it was his idea and Ned is well off enough that he doesn't even need the money). Of course, Peter and Ned are friends the same way Ferris Buller are friends (kinda one-sided). Things do kinda deteriorate throughout the film though, the starting point of this being when Ned's recording drone (which was Ned's idea that he paid for, btw), is destroyed in that bank robbery with the Fake Avengers.
3. Vulture. Peter and Adrian, if not immediately knowing who each other is, would definitely meet earlier in the film, him and Liz actually dating (or maybe Peter and her do homework together). She invites him for dinner, he offhandedly mentions the "Stark Internship," and Toomes goes on a less-hand-showing but still passionate speech about how Stark has never had to "really" work a day in his life, and how his actions don't do anything to protect the little guy. Peter asks isn't it important that Stark still manages to save people still a god thing, only for Toomes to snide that Peter "doesn't understand how the world works (this would be a repeated line throughout the film.). Basically, this is a dark mirror of the dinner scene between Peter and Otto Octavius from Spider-Man 2.
4. Iron Man. Still a limited role appearance but his first appearance now takes place after the equivalent of the D.C. scene (which happens much earlier), which consists of him telling Peter to watch himself, because people might come to get him to sign the Sokovia Accords, to which Peter thinks is great, because it get him closer to being an official Avenger, only for Tony to tell him "you don't know how the world works." This scene also ends with Peter finding out Vulture is Toomes.
5. Big Gamble. After getting chewed out by Stark and "becoming a hero," Peter is agitated because, hey, he's a smart guy and just wants to provide for his Aunt May, dammit. At this point, Peter and Ned are on thin ice, and Peter actually considers going Vulture in order to make a big profit (which he does). Ned decides to get his second, newly acquired drone to figure out where Peter is going. This leads to the drone being discovered by the Tinkerer, Ned realizing he has nowhere to hide (in hindsight, trying to hide at the Statue of Liberty wasn't a great idea), aerial fight that ends up on a boat (Spider-Man manages to damage a wing and get Vulture to retreat before they get to the Island), but Peter, and damaged alien tech. Boat scene, Stark comes in to save Peter's ass, manages to make Ned's tech untraceable, but Stark takes away Peters suit, BANS him from being an Avenger, and Ned and Peter's relationship is shattered. Oh, and Stark remotely accessed Peter's suit so that May learns everything, too.
6. Act Three. Peter's social and career life is pretty much over. May and Peter have a talk about why Peter did what he did (MAy's money troubles), only for May to tell him what Uncle Ben really would have wanted from Peter, and hat he needs to take things slow and understand how the world works. After school on Homecoming Day, Toomes actually comes and helps Peter get prepared to take Liz to the dance (he's figured out who Peter is, so basically he's kidnapped Peter to show Liz a good time). From there, the movie basically proceeds as normal, only after Vulture drops Peter and Liz off and gives Peter his ultimatum, we actually do flashback to when Uncle Ben was shot (it was over something trivial like in the Amazing series which Peter witnesses, but it was an accidental murder as was revealed in Spider-Man 3). Now, when Peter's trapped under the rubble, he's confronted with all the bad things he's done through the movie, and he rises above them when he lifts the building up (so now that moment actually means so much). And when he turns down Tony, he has become more responsible and decides to take things easier, smaller.
Life moves pretty fast, but if you don't stop and look around once in a while, you could miss it.
7. Pepper Potts. Now, I did spoil myself a little before seeing the movie, so I knew Pepper was going to be in the movie and Tony was going to propose to her, but they way I read it, I thought she as part of the press conference crowd and Tony singled her out, and his asking Happy for the ring was him deciding to do something spontaneous and in the moment. We don't actually see him doing it, so it leaves the question of whether he's successful in winning her back.
So there's what I would have done differently, and I hope I havn't upset anyone with a borderline unlikable Peter Parker (hey at least the movie would acknowledge he's a jerk. Kinda like how the first Iron Man knew Tony was a jerk pre-shrapnel).