I'd like to see altitude / energy management with countermeasures and deeper missile play. doesn't need to be flight sim level but it'd be cool if the upgraded planes gave you more than more HP + more DPS. Also, we could use sidegrades to the different planes, with missile carriers vs. gunfighters (or mixture of both), plus a loadout system that gives the aircraft greater range and speed in exchange for a lighter loadout (less missiles / guns / countermeasures)
If I remember X1 had some aircraft sidegrades, F16/Mig-25, I think that would be relatively easy and desirable to put back in. If anybody cares about my ideas I could sketch it out more thoroughly but that's what I want most of all.
Oh, and I'd like the ability to escort transports, if that doesn't already exist (maybe it does..)