Off-topic flag should allow “other” choice text entry
The 5 sites listed in the off-topic site list cannot encompass all the possibilities for off-topic questions on SO.
While accepting the reasoning behind the Don't make me think barrier mentioned in the answers to the post More options when flagging for migration, there is a healthy compromise which allows users who want to think to suggest appropriate target sites.
Similar to how the Close as exact duplicate UI suggests a list of similar topics as well as providing an autocomplete text box for users to suggest other options, it might be useful to keep the current list of 'most likely target sites', but to also add such a text field where users can suggest a more appropriate destination.
This will prevent useful, insightful, and well thought out questions, which have simply been asked in the wrong forum, from falling between the cracks, so to speak. This is also obviously simpler for users than having to flag for a moderator's attention to suggest the duplicate, as suggested in this answer.
The place where I see this feature being most useful is between SO and math, though there are several obvious instances in which this feature would be helpful.