Flash doesn't make sense because he died way after Mysterio was already resurrected, whoever Kindred is, needs to have died before Mysterio came back.
Uncle Ben, you'd need retcons to establish that he has a connection with Norman, even then, there's Norman talking with Kindred and he says that Kindred is part of Norman's legacy, and that he's proud of Kindred, there are few characters who fit this, Harry (And Norman would need to suddenly be proud of him) and Gwen (Not proud of her as a person, he's proud of having killed her to make Spidey suffer), maybe there's someone else, but those two are the most obvious.
Honestly, after ASM#31, I stopped believing it could be Harry, it was too obvious, I mildly considered Gwen, but I just wasn't sure, mostly because Kindred is showing a lot of hatred for Peter, and Gwen loved Peter, but hated Spider-Man, it's hard to say which one between love and hate would triumph if she learned his secret identity (And Gwen's clone from Clone Conspiracy showed it was love), but I was thinking that she wouldn't become someone who's killing innocents even when hating Peter, but with Gwen getting a mini now, maybe they did that simply because she's one of Spidey's 3 big love interests, and Felicia and MJ got it, so maybe it's her turn just because, or maybe it's because of Spider-Gwen, or both, but I'm seriously considering that this mini is going to be a stealth prequel to explain how she becomes Kindred, either that or the timing of this is a huge coincidence lol.