1. I'd personally like to see Boo, Toadette, Dry Bones and Blooper return. I like all of them, but if anything Boo should definitely return. DK would be the top choice for most, and I wouldn't be surprised if he returned, but I'm personally enjoying the roster without him at the moment. =p
2. Definitely Shy Guy. I'm not sure why it's taken so long for him to be playable but now that he is, and because he has so much history, he should stay.
3. I hope we don't see Koopa Kid, Kamek, or Birdo again. Koopa Kid is a thing of the past (Bowser Jr. has taken over), Kamek is too bland and Birdo needs no explanation anymore.

Also I'm tired of Koopa simply being invisible to most and taking a valuable spot in the roster so I'd rather he not return or be replaced permanently with Dry Bones.
4. That's one possibility that I'm 100% certain will never happen. However, because Mario Party 10 should be the biggest of all Mario Parties, I wouldn't be surprised if everyone returns with the the addition of two more.
5. Rosalina playable or not. She'd make the best host out of anyone because a Mario Party in space, more specifically her observatory, would be awesome. If she's not the host, it would still make a very nice playable character who I'd personally all the time. Star Bunny would also make for a new interesting choice.
But yeah, Mario Party 10 will be the hardest to predict what will happen. It will probably either be one of the best, or one of the worst, and I'm hoping they spend a lot of time on it to make it the best it can possibly be.