Instead of an app like Google Flights, Hotels.com, or multiple train/bus sites, why not:

Use a future AI travel planner that searches all the hotels/hostels/Airbnbs, plane flights, and train/bus connections as well as car rentals (if you so desire), and then posts out your entire travel itinerary based on the lowest prices for the travel combination based on the locations selected on the days or even months you have desired to travel and with the amount of days for each location.

What are your thoughts and would this type of startup become unicornic or would it just fail to oblivion?

Airbnb, Booking, etc, became successful however.