Ninja Gaiden Black was amazing, i just finished hard mode last week.
Ninja Gaiden Sigma is coming out for PS3 on June 26th. From what i understand, its just a much better looking version of ninja gaiden.
Rachel will be a playable character, and much like Vergil in Devil May Cry 3 S.E., she will have her own isolated story path. Using her fiend hammer/war hammer (...lol pun) will be much more agile and nimble than when Ryu retrieves it in (i think) Chapter 12 of his path, after beating the Greater Fiend Alma V1. This works out though, because rachel was always working right alongside Ryu in the main game, but Ryu only caught up with her when their paths intersected. If i remember Ryu's patch correctly, Rachel starts off from a village, goes to the city, fights doku, fights her sister in a cg cut scene, fights doku again, gets captured, and her last real important appearance is atop the Pyramid when fighting Alma 2.0. She shouldnt kill Alma in her story path, in fact, since Ryu kills most of the important people, her bosses should be mostly unique to her, the story paths arent supposed to conflict, but support. Rachel will be able to use weapons other than her hammer, but she will not be using the dragon sword, plasma saber, or any of ryu's key weapons. I have no idea about her projectile weaponry, if she has any, she will be fighting fiends for the most part.
Ryu will pretty much be the same, a few new acrobatic movement commands have been added, including faster canceling between attacks. There will be new enemies, mostly fiends and more variations of old enemies. Some weapons will have updates as well (the useless smokebomb... maybe itll be the most powerful... jk). Overall the game is going to run even smoother than before. What i dont know is wheter enemy placement will have a "Black Mode" or if it will be unique. Not much is changing from the core game, mission mode may/may not be imported from black. Mostly, im wondering wheter Sigma is working off of Black or the Original (XBOX Title)
It's also worth noting that Ninja Gaiden will be getting a DS release. Full touch screen controls, 3D Graphics, in an interview the creator was talking about his fascination with the "touching" part of the game. Supposedly the only button used will be the
buttons on the front, whenever any one is pressed, Ryu will block. However, eliminating input commands will probably take alot away from the game. I dont have much hope...
Right now, the game is tapping into its previous popularity reserves, and are moving forward. This is all leading to the amazing release of Ninja Gaiden 2, unannounced, unofficial, but will exist sometime. (Ninja Gaiden 2 is being worked on, btw, Tecmo reveals some of this in DOA.
Speaking of DOA, Ryu pwns. Hope this helps.
"Thats the problem with strengthening, if you try to change something that is already perfect, you risk making it imperfect, so why take the risk?" ~Emiya Shirou, Fate/Stay Night