Update: It looks like the Peter Stormare rumours were indeed true - See this LINK in the Spoilers section. This along with the fact that Harold Perrineau was signed up leads me to suspect that this is a great source I've picked up and hopefully the Forest Whitaker rumour is also true.
One of my contacts at ABC has provided me with the following interesting casting news. This source has proved accurate in the past but he assures me this is legit. As I've not been able to verify this with my sources as of this time, I have to post this in the rumours section.
1) Negotiations with Harold Perrineau and Malcolm David Kelley are being made with Malcolm contract practically agreed and feelings are that agreement with Harold's people is very close.
2) Only one new actor will enter in the fixed cast, and the actor currently being sought for this role is Peter Stormare.
3) The producers have invited the actor Forest Whitaker to participate in two episodes at the start of the season.