If you’re going to kill off a character in a sitcom, you really need to have it create a ripple effect. Since there’s been very little interaction between Phil’s father and the Pritchett branches, I don’t think his death would push the envelope enough. Ditto with any of Haley’s many boyfriends. Pepper’s death would only bother Mitch and Cam, and I’m not sure Javier’s death would even upset Manny at this point.
Some of the clickbait sites say that Ariel Winter is leaving, and imply the line between that and Alex’s death, but killing any of the kids would be terribly dark.
Put your quatloos on DeDe by process of elimination. They already noted she and Claire have the same heart condition, so the death certificate is all but signed.