Okay I've just gotta say, next to Garrus Kasumi is my FAVORITE character ever. Despite this though, I have a strange feeling in my stomach that Kasumi will be a main character in ME3...
DLC seems to be important to the ME franchise (heck they told the story from ME2 to 3 with Overload to LOTSB to Arrival all through purchased content) so I don't think she'll be scrapped due to being DLC. She was obviously supposed to play a big part in the game (and if she was supposed to be unimportant, why make the graybox and her loyalty mission tie into the storyline so much? why not make it simply to gain loyalty, like Zaeed's?) She's gaurunteed to make at least an appearance in ME3, it's been confirmed. I, however, think she'll play a very large role this time around. And if she does, then she wouldn't be the first DLC character to make a sequel as a full character in a BioWare game. (Anders anyone?)
If she's a main character and you didn't download her, she'll probably play out like Garrus if you didn't recruit him in ME1 or Wrex if you never met him. She'll probably seek Shepard out herself and ask to come along as opposed to her reminiscing with him about the suicide mission and the "good old days". If she died, then she simply won't be there along with Garrus or Tali. But this also puts Kasumi in an interesting position. She's an old face from the first game, yet hardly anything is known about her. This gives us plenty of time to get to know her all over again with interactive dialogue in ME3, and I gauruntee she'll skyrocket in popularity once she feels like a full character. From a BioWare standpoint, this would be a good strategy as you keep the fans you already made with this character and gain even more as you go into depth about her. Which is only magnified if she is a romance capability.
Which also leads me to my next fact. There are now same sex romances in ME3. While Kasumi was with Keiji, her sexuality is never truly defined. With a little tweaking, Kasumi could become a prime member of the new same sex community, which will be good news for Kasumi lovers as it would cement her inclusion in ME3. Again, that "old face with new features" position comes back around again. And of course, she's super hot, so the MenSheps will totally be all over dat ass.
Her powers are unique, she's one of the only tech/infiltrator types who can turn invisible and would balance out our team nicely. She could interact with the team greatly (she's done business with Liara, knows Garrus, loves Tali, Ashley could see her as a liability if she kept the graybox ect.) and with her personality of being a sentimental risk taker, I can't see her turning her back on shep when an adventure like this is right under her nose. And her personality is so rare in Mass Effect, we don't often see a blind optimist with cute dialogue come our way =) not to mention with all the new types of fighting (such as being able to take control of mechs and the new mounted turret firing sections of the game, a stealth mission might be a nice addition to the fray?)
Also, I've been wondering something... after the events of ME2, the Normandy has been taken over by the Alliance who claims it to be theirs. They are taking the ship apart and upgrading it while Shepard is on trial as a wanted criminal. However, it is told that all of Shepard's guns and personal affects are still available to you in ME3... which then the question was asked... who has them? Is this the work of our favorite asian thief? =) Maybe it's just me, but I could totally see Shepard escaping onto the normandy and going up to his/her cabin where all of his/her stuff is placed perfectly in the room. As he/she questions how it all got there, Kasumi uncloaks and remarks on how that recovering all of your items might be her last job as a thief. She then asks to join you on your final mission and off you all go to save the galaxy!
I don't know. Despite me loving her so much, I just realized that the more I thought about how she would fit into ME3, the more reasons I found that make it such a good idea. I totally support Ms. Goto and I guess all we can do now is wait and see =)