Mary Jane Watson partially taken over by the Venom symbiot
oooh that would make one secy spiderwoman... and deadly. Spidey killed by the love of his life , oh drama.
Or maybe, Peter *seemingly* dies and Mary Jane's pain, sorrow and anger call the symbiote to her. The suit, having once loved Peter bonds with her and they use their combined pain, anger and sorrow to hunt down the one who killed Peter only to find out later that Peter is alive, causing conflict between her and the symbiote.