Let's say Gwen Stacy returns for future sequels...

None.

 

I guess in Spider-man 4 Gwen Stacy returns, i think 4 and before spider-man 3 was known that spidey killed Gwen's father ,there was a fight with green gobin 2. Or Spidey fight green goblin 2 have ball pumpkin throw out bomb build just broke fall down and Captain looks out at kid falling down , then he runs and pushes the kid away, then he looks at fall he is dead just crash all spidey is fault. Gwen cries about spidey killed her father. Peter fights with Green Goblin 2 is not dead just in hospital before 3 to be continued. Then in the future Spider-man 4 Returns Green Goblin with Gwen fights with spidey this battle on the bridge ,then he throws bomb to Gwen falls down to the river, then spidey spins web and catches a leg of her.Then spidey looks in her eyes dead and screams " Noooo! "She is dead. Is this going to happen in future spider-man 4??

 

she can turn into Carnage.