It's called Retiring.
Super-heroes don't retire. Well occasionally they do but it never lasts long. Something always happens to drag them out of retirement.
However as in the comic, Bucky and Falcon have all taken over the Captain America mantle, it is possible to kill Steve Rogers but have the alter ego survive. Same with iron-man. Tony could step away and become an Alfred type support character to a new person in the armour. Or be really ballsy and kill RDJ. And even Thor has had other people wield Mjolnir (my favourite being Beta Ray Bill).
They could be interesting and kill Hawkeye's family (as they did in the Ultimates comic-verse) and have him go on the rampage before redeeming himself on a suicide mission. And Vision dying would be the catalyst for Wanda going crazy warping all of reality. Sooooo many options