I just hope that if you saved the Rachni in the first game you get a rachni "dog" in the 3rd game. That is, a character like the mabari from Dragon Age.
But speaking of characters, what of all the characters that survived the suicide mission? Some like Jack, Zaaed, Kasumi, Samara/Morinth, and Grunt would have likely excuses to leave. But what about Johnson, Miranda, Garrus, Tali, Mordin Solis (thought I read somewhere that he's already been confirmed as a returning character), and Legion (he had better return, you only get to use him for like, 2 missions before the game ends and if you downloaded Kasumi the only way to have everyone survive is to not activate him until after you've completed the main quest)? Everyone in the first game abandons Shepard because Shepard done gone went and got spaced. Will they have another such excuse for Shepard to crew up from scratch? Or will we have a full load of characters from ME 2 as playable characters in ME 3 along with a fresh batch of characters to recruit?