I think Apple's current approach, where all the smarts (Machine Learning, Differential Privacy, Secure Enclave, etc.) reside on your device, not in the cloud, is the most promising. As imagined in so much sci-fi (eg. the Hosaka in Neuromancer) you build a relationship with your device which gets to know you, your habits and, most importantly in regard to search, what you mean when you search for something and what results are most likely to be relevant to you. An on-device search agent could potentially be the best solution because this very personal and, crucially, private device will know much more about you than you are (or should be) willing to forfeit to the cloud providers whose business is, ultimately, to make money off your data.