I would say raina is a better option but i dont think some one will replace dhoni because he is doing a quite good job,i am expecting him to lift th 2011 world cup...
I think it will be a long time before someone to replace dhoni. Doubt BCCI will think of replacing him giving the stats that team has improved a lot under his cap.
Nobody can assume the remaining working days of maahi. Since he has some back problems and back problems can be hazardous. also, he plays all the three formats of the game without any rest. He does wicketkeeping, batting and captaincy. so, It can become tough for him to manage all these things for long. as far as the next captain is concerned, raina, kohli and rohit sharma can dominate the discussion.
Harbajan singh is a good option after dhoni because he has good experience and also some leadership qualities.
Raina and Kohli sounds good and practical. Rohit Sharma? = Wasted Talent (He needs to get his place in the team first, then we can think about being the captain)