Well, for one, MI 2 seems to be a sore spot for Cruise himself, and he appears to try and ignore it as much as possible. As for Nyah, Thandiwe Newton is on record about how unpleasant her work with Cruise was, specifically with their preparation of the balcony scene. She did note however that he tried really hard, so she might not blame him. Doesn't mean she wants to work with him, again, though.
I would have loved for them to get Jeremy Renner back, though the timing of the production and his accident obviously would have prevented that even if they had tried to make it happen. But Paula Patton would have been great. Renner would have made more sense, though, as he could have been one of the people around Sloane instead of being on the actual team.
But obviously, getting actors and their characters back is a slippery slope, as there just have been so many over the years. A case could also have been made for Maggie Q, Jonathan Rhys Meyers, John Polson, Anthony Hopkins or Laurence Fishburne, and that's just former IMF people. And at some point it would have become just a "'member that character?" situation. With Kittridge and Donloe, they already had two of those, and they were smart enough to actually give them something to do, that would have been difficult with even more characters.
It's good that instead of getting the movie too over-crowded, they made more references that helped the plot of the movie.
Like the Rabbit Foot reveal, I didn't think we should ever find out as no answer could have been satisfying, but you know, I was actually satisfied, because they made it have emotional consequences for the characters.