Here's one I'll never finish, but wanted to....
The trend in gross revenue per flight hour among traditional airlines and LoCos world-wide over the period 1990 - 2005, seen against the trends in DOC's world-wide; ie what was the money spent on, in what proportions, in 1990, and what is it spent on now, in what proportions.
I did enough on this about two years ago to reveal some very startling information; seat capacities and load factors went up while fares went down, and gross revenues per flight hour fell dramatically. At the same time fuel and crew costs went up, as did airport charges, on route charges and handling. Ownership and insurance costs went up, by and large, although clever lease & buy back deals offset that. Depreciation was extended by the bean counters to unrealsitic periods (50 years in one case) to make things look better.
What's left? Why, maintenance, of course. Parts went up, leaving maintenance labour as the only cost that could be reduced to a fraction of the share it used to have of income, without anyone noticing. So fully-qualified C licensed engineers are working at rates that would make the local garage owner blush. Unlicensed fitters and mechanics, with good experience and qualifications, are getting the same as road sweepers.
And that's why the biggest challenge the industry faces, after environmental concerns, is how to manage when the last well-trained engineer has locked the hangar door, switched the light off, and got a job on the railway.
I hope I've whetted your appetite; things have moved on but there's some really interesting data locked away there to dig out and analyse. What's more, no-one else has really done it, as far as I know, and the results of the research could have a real impact.
Sorry, that turned into a rant, almost! But it is fascinating.