I'll make a spoiler block of it all, since this can be a spoiler to anyone who hasn't seen/read the Pain and the Five Kage Summit arcs.
After Pain's attack on Konoha, Tsunade exhausted her power and Danzo was nominated as Rokudaime Hokage. This lasted for a very short period (Danzo died fighting Sasuke) and Tsunade soon recovered and was, once again, appointed as the Hokage. So next will/should come the Rokudaime again and Danzo should be ignored, or should we move on to the Shichidaime/Nanadaime (not sure about this) Hokage?
I ask this because some people and sites consider Danzo only a temporary Hokage (in this logic we're still missing the 6th, so the next should be it). However, something similar has happened with the succession from 3rd (Hiruzen) to 4th (Minato) and back to 3rd (and later came Tsunade, the 5th). From this point of view, the next to succeed should be the seventh.
Does anyone have any thoughts on this?