I never liked Blooper in MP8 (no offense to Blooper fans). It just seemed so out of hand and random to me as well. I don't mind Hammer Bro. as he seemed to fit somehow to me. I guess it was the lack of a humanoid body shape that makes me not see Blooper as a playable character. So I would prefer if Hammer Bro. reappears in MP9.
I actually want all of them to come back. Koopa Kid (Unlikely), DK (Unlikely), Blooper, Hammer Bro. and Dry Bones (Extremely Unlikely). I want all them back before they start adding new characters, but thats just me.
I did like Blooper a bit though, he was unique compared to the others. Koopa Kid was like a Playable Bowser to me, since they resembled each other. Hammer Bro, I havent got him yet, I hired out MP8, just had to beat him in a Board and beat the Last Minigame but it was due back and I couldnt get him :P. DK, I would like to see him back, just for Nostalgia reasons. No Idea why they had to make Dry Bones get replaced by Koopa and/or Shy Guy, but them two will do fine anyway.