If Jeff does leave I would not be surprised to hear the series would end. He is more than just a host. He and Burnet work closely together; it is very much a team effort. Jeff has already said that there is nothing more new to do. Better to go out and leave em wanting more. Burnet will create another show.
If he is replaced, only an unknown would work. Hatch is too controversial. The host has to be the glue and ground the show. We cant have any preconcieved opinions about one to take over this position.