Almost certainly multiplayer battles, individual PvP, P+wingman PvP, multiplayer PvP. It's the next logical thing to test now that we are used to the interface and controls, and most importantly it will test the peer-to-peer mechanics of the game. These are really core things that have to be right from day 1.
I wouldn't be at all surprised if the alpha test group never see some parts of the game and they go straight to beta testing. If FD keeps on target with releasing the game at the end of Q1 2014 that only leaves 3 months for the rest of alpha, beta 1 & 2, and the promised gamma!
If the trading system, for example, is considered by the devs to be robust and not in need of a lot of testing then it may not be seen until beta 1 or 2 - same with mining. Combat is the big player interaction aspect so I imagine the alpha will probably focus strongly on it to the exclusion of other areas. I could just as easily be wrong, but I bet if we do see all the game in alpha a lot of the modules will be released in one go right at the end of the process.