Boris Johnson would, of course, be very different from Cameron. But as Matt notes, if Boris returns to Parliament early it will be regarded as a declaration of intent.
Boris’s greatest weakness is that he’s seen as great fun but not a man of substance. The best way for Boris to deal with this problem is to serve two successful terms as Mayor of London so that he can point to his experience in the capital when asked about whether he’s up to the job.
Speculating about Cameron’s successor at the moment really is just a bit of fun; as Matt says it is entirely possible that Cameron’s successor could be someone who has only just entered parliament or who isn’t even here yet. But it is one of those subjects that gets people talking.