There's a lot happening on the MRO (Maintenance, Repair, and Overhaul) side of things... a lot of it to do with entering and emerging from bankruptcy. Some items that come to mind are Delta's dealings with parts supplier contracts while under Chapter 11 protection (having greater ability to renegotiate and break contracts). Delta has also ramped up a lot of in-sourcing (CFM56 and CF34 engine overhauls along with associated components) as an attempt to increase revenue through TechOps. Northwest also completely outsourced nearly all component and heavy maintenance.

A lot of the MRO discussion would be directly related to cost-cutting attempts, how well they worked, and savings vs. value. You could also look at cost-cutting as a whole and what it has done to airline service level (an interesting comparison would be domestic vs. international and US vs. European carriers).

Outside of MRO I would look at the emergence of code-sharing and airline partner programs, especially the regional airline partnerships and how they play in to the hub/spoke system.

Another interesting topic could be mainline-legacy carriers compared to low-cost and ultra-low-cost carriers. A historical look at Ryan Air's emergence in Europe and comparisons to the US low-cost carriers and the very few US ultra-low-cost carriers (Spirit and SkyBus come to mind) would be another good study in the US vs. European aspect of air travel.

I'm not sure what your research time-line is like, or how specific your topic needs to be, but these subjects could be comprehensively covered using readily available research materials. Most of these topics and trends are well documented within the airline business, but not overly discussed outside of it. Aviation Weekly and SpeedNews are two great publications, one pay and one free, that could give you a lot of insight into the inner workings of the airlines that most publications outside of the industry will not.