I wonder if a travel-planning-app-as-a-service would work? Or maybe more as an affiliate system. I.e., you build a travel planning app and try to convince the OTAs (and the airlines, but I don't see how) to sign on as affiliates. Your travel planning software might have to be embedded in their site, I don't know. It might not be workable but it does dodge the user acquisition/retention problem.
It's more about the "how" than the "what". If you have a general idea about what you want to do the trip planners give you unbiased advice on how to do it and save you maybe 3 or 4 weekends of trip planning
