Seriously Gwen's death makes so many plot points work. Just off the top of my head
-Peter/Goblin revenge story, Peter revenge story all up. You could have him going after all the people that killed the people he loved, that is, both Goblins (Norman and Harry) and Ben's killer
-Myterio: Imagine this, could be an emotional rollercoaster, if people were fine with mysterio being retconned slightly you could even make him a primarily pyschological villian, who is just hell bent on using the weakness' of others against them, clearly Gwen's death would just be huge as a catalyst to plunge Peter into some sort of depression.
Obviously the black suit ties in to all of these darker themes, I could really see a dark spiderman film happening, working and making sense contextually for the first time on screen. Basically Peter sad=Black suit.
I'm pretty pumped for the next film if they go on this dark path, it could actually be something everyone could get behind. Also is a catalyst for MJ to make her debut in the series, as the light at the end of the film that drags Peter out of his rough patch, makes him ditch the suit, it finds venom (flash?) and bang, TASM4 is all set up.