The Sam pack and the Blade Wolf pack
The first pack is called "Jetstream," which allows you to take control of Sam, Raiden's rival. The other will be a side story featuring Blade Wolf. The packs are said to feature "multiple hours of gameplay and unique abilities, while expanding their storylines and origins."
I'm not expecting the world from these packs in terms of deep stories, but I'm sure they'll be extremely fun to play. Expect our impressions in April!