This question already has an answer here:
- What is a “locked” post? 1 answer
It's well known on SO that some questions are essentially impossible to delete despite being a bad fit for the format, because of their sheer age or popularity. Essentially, they will become a part of a game of tug-of-war where they will be repeatedly closed and then re-opened, never allowing deletion to happen.
What I'd like to propose is a kind of reinforced close vote.
Once a question has been nominated for deletion, people with enough reputation to vote to close a question can elect to 're-inforce close vote', costing them one of their own close votes. For each person who reinforces the close vote, it will take 1 more vote to re-open the question.
This seems like it would allow a good balance between being able to re-open existing questions which have been edited to be a good fit, and allowing a deletion vote on a popular but unsuitable question to go through.