What would be really interesting is an alternate universe story, where the Daleks do manage to wipe everything else out. They do begin to self-exterminate down to a point, kill Davros (for good). But get bored.
They then bring back all the species they had killed (to subjugate) needing an audience--and then startback on the warpath all over again.
The sad thing was, the new Earth they created to take over was actually nicer and more decent that ours.