I'd expect another smallish cluster of players to appear as early as tomorrow, but sometime soon in any case. I would not expect many prospects in that list, however; they are primarily veterans that were not present either due to lack of contract or long-term injuries. Prior, Nick Johnson, Bonds, Clemens, Corey Patterson, Ensberg, some others that I don't remember at the moment.
For those wanting prospects to be added ASAP, I'd set the general expectation that, especially once the season starts, we'll be very reluctant to add prospects unless their arrival in the major leagues is imminent. There are two camps on this issue, and going forward we'd like to be able to make both happy, but until that point in time we're likely to err on the side of not introducing prospects too early during the season.