It shouldn't even be a "hard" problem any more. Search and relevance are mature technologies. You do need to provide some features, but there are many: time spent interacting with tabs, was it a "terminus" in a navigation session, active user marking/tagging, signals from other users, other products, etc. It would be an excellent use of ML. The rest is capturing these in a user friendly product, though if it makes Megacorp no more ad money, I doubt anyone will work on it however useful it may be.