Quite the question, since I'd love to have a site that automates what I otherwise will spend hours doing manually:
- Start with a departure city or range of departure cities.
- The ability to pick a destination and have the site run a recursive search from the departure airport(s) to every possible destination, recursing down to repeat the search for those airports, returning after a preset number of max connections on potentially (but not necessarily) separate tickets to list all possible routings from departure to destination with associated total cost (give or take baggage charges, etc) listed nearby.
For example: I wanted to book a flight from ORK to POZ, but balked at the €250 cost for a direct one-way. After some looking, I realized I could route ORK-MUC-POZ on a combination of airlines to get a one-way for €100. For a few more connections, a RT via DUB, ZRH, and TXL would have come out to somewhere near what the direct OW would have cost.
- The option to link nearby airports by non-flight transport would also be nice. Just in case a large savings can be made by doing a transit from, for example, TXL to POZ by train rather than flying directly.
- The ability to input an origin and budget and return a list of possible destinations, filtered by carrier/alliance of preference.
For example: I have X days off and €300 to spend on flights. Where can I go?
- The ability to only return a confirmation number would also be great, but that's only for people who like a surprise when they check in.
"So you're going to Athens today"
"I don't know. Apparently!"
- The ability to white-/black-list carriers. I want to find a trip to X, but I'll walk before I fly FR, for example.
- The ability to specify booking classes. I love how many sites will automatically break it down to show cheapest, flexible, etc. But I want one seat in discount Y and one in slightly-more-than-discount Y, as I need status miles but my co-traveller needs the cheapest fare.
- A column listing the mileage accrued on your FFP(s) of choice per fare class. Ideally one listing the total mileage for each routing as well.
- Although it may only be applicable for a few, the ability to book an itinerary but restrict it to as close to a predefined itinerary as possible. Alternately, to specify two itineraries and force the booking engine to keep them as identical as possible.
For example, I book one itinerary for my girlfriend to go to a conference, with me meeting up at a later date and the two of us flying back together, potentially to different destinations. Things like forcing overnights where one overnight might be forced already, etc. We do this every now and then, and it's just that added bit of effort to try and coordinate as much of the journey as possible.
- A "minimum miles" routing tool would also be helpful. For when you need a MR and just specify "show every routing that is more than x thousand miles".
I'm sure I'll come up with something more, but there's lots of web tools I'd love to see for this!