New to ACX is a collection of tunable aircraft. As you soar through each mission (mostly blowing things out of the sky, plus a handful of defender tasks) you'll unlock new parts and accessories for your new, unique jets. These aren't typical licensed fodder, either - they've been invented just for this title, and they're all yours to pimp how you see fit.
There's also a bit of choose-your-own adventure action going on. The way you behave in each sortie can affect other missions down the road - do you tag that escaping bogie or bail your comrades out of an untimely, fiery death? Your choice will dictate the next objective, and possibly piss off your charred friends if they die.
Multiplayer's never been a big draw to the Ace franchise, but the PSP game's 4-player, ad-hoc matches could easily change that. The fourth and sixth games had 2-player split screen, but now you'll get your own view of the mid-air missile barrage. We're excited as hell to see how well this franchise is holding up, and things are already looking pretty sharp for the latest trip through the horizon.