A user field type that will link to an existing topic. The user will select the topic from a searchable list, just like when an admin is moving messages to a new or existing topic:
Once a topic is selected in User Profile, it will be shown as a link to a topic. The link’s text is the topic name.
Might be a plugin territory, yet not sure — linking to an internal topic, rather than something outside of the forum, also feels like a native feature.