I've heard the online will have unlockables, but I can't find any details on what they will be. Are they all cosmetic unlockables? Are there longterm goals, or would most players unlock them within a week of playing?

I'm fairly sure secondary weapons can be unlocked e.g Uzi rather than standard machine gun. Although I have no idea how long this would take or whether this would be the only type of unlockable. I certainly hope not as, for me at least, unlockables can really keep a game fresh over time.

Every rank you hit online gives you 1 unlock point. You can use that point to unlock a car, gun, killstreak bonus, energy buffs or car skins. There are different tiers for unlocking things and you may have to unlock everything in the first tier of a section in order to unlock the next.