I need to write a new epilogue for Tuck Everlasting, and I am confused about what to write.

Students should love this kind of question, because it is completely open-ended. You could literally write just about anything. 

The current epilogue takes place after Winnie has lived a long life and died. Angus and Mae Tuck have returned to Treegap in order to see how the town is doing, see the spring, and find information about Winnie. The woods have been bulldozed, the spring is gone, and Winnie Foster has died. Angus and Mae...

Students should love this kind of question, because it is completely open-ended. You could literally write just about anything. 

The current epilogue takes place after Winnie has lived a long life and died. Angus and Mae Tuck have returned to Treegap in order to see how the town is doing, see the spring, and find information about Winnie. The woods have been bulldozed, the spring is gone, and Winnie Foster has died. Angus and Mae see her tombstone and realize that Winnie was married and had at least one child. They then leave the town.

You could write an epilogue that takes place while Winnie is alive. She could still be married. She could still have kids. I would make her a middle-aged mom, and I would have her sitting on her porch reminiscing about the Tuck family and wondering what happened to them. I would have her, in her thoughts, mention that she is glad that she made the choice to pour the spring water out on the toad. The woods and spring could even still be bulldozed, and I would make Winnie completely content with that.

You could have your epilogue be focused on Jesse Tuck, too. You could make him angry at Winnie, or you could have him be glad that Winnie was able to move on and forget about him. You could have him be the one returning to Treegap. 

You could time your epilogue long before Winnie's death, and you could write about a time 30 years after the Tucks' departure. Winnie and the Tuck family could be continuing the tradition of getting together every ten years during the first week of August. Your epilogue could be about one of those reunions.