I have an existing question where one of the answers made, got me wonder about another thing. Should I revise my original question, adding the new question, or ask a completely new question?
That would probably depend on the answers that have already been left. If you change the question so that the majority of existing answers are then irrelevant that would just be confusing for any new visitors reading it. However if there are no, or few answers and your change will bring the question more in-line with the answers left then I see no harm in making a change.
The thing to consider is that people come here from web searches primarily, and they're likely looking for an answer to a specific question. If you can change questions to solicit better responses and to fit better with how responders interpreted the question initially then you're improving the overall quality of the content and that benefits you and any new visitors.