As we all know by now, the Legacy will allow you to unlock races for all the classes once you've reached level 50 with said race.
From what I could find out from the character creation screens, there's 3 races Empire-only, and 2 races that are Republic-only.
Seeing as to how the Legacy is currently limited to one server, you might want to think about which races you pick, and which you will want to unlock if you want to have a special race from the other side of the fence, having all 4 classes per faction.
Sith Warriors: Human, Sith Pureblood, Cyborg, Zabrak
Sith Inquisitors: Human, Sith Pureblood, Twilek, Rattataki, Zabrak
Bounty Hunter: Human, Cyborg, Zabrak, Rattataki, Chiss
Imperial Operative: Human, Chiss, Cyborg, Zabrak, Rattataki
Empire Exclusive: Chiss, Sith Pureblood, Rattataki
Empire Limited: Twilek
Suggested race picks: Sith Pureblood, Rattataki and Chiss are Empire only. Consider that if you wish to unlock them for the Republic.
Jedi Knights: Human, Miraluka, Twilek, Zabrak, Mirialan
Jedi Consulars: Human, Miraluka, Twilek, Zabrak, Mirialan
Smugglers: Human, Cyborg, Twilek, Zabrak, Mirialan
Troopers: Human, Zabrak, Cyborg, Mirialan
Republic Unique: Miriluka, Mirrilian, Twilek
Suggested racepicks: Twilek is pretty uncommon for the Empire to have, and Miraluka and Mirialan are currently Republic only.
Human and Zabrak are available for all classes, and thus not recommended if you wish to unlock new species.
Cyborg is pretty common, but still limited to some classes, so worth considering to make one of them as well.
Twilek is also recommended to make as a republic character, as they have more options for them.