Admittedly, the greatest difficulty in approaching this question from the perspective of an advisor and tutor is the lack of additional and specific criteria, combined with the abundant amount of options we can imagine this task exploring. For me, my thoughts immediately go to Macduff's function in the play; should we see him as an independent character who happens to be the "good guy", or an embodiment of morality that fulfills a function within the plot, for example his purpose is dictated by the dramatic structure of the story? Other possibilities along this line would involve asking how much our own understanding of Macduff may or may not align with Shakespeare's vision of the character, and what it means for us to confirm or reject the character as he exists within the canon of the text.
As far as what is "left to live for," after defeating Macbeth, Macduff is still thane of Fife, as well as an earl, "the first that ever Scotland / in such an honor named," as bestowed by Malcolm in the final lines of the play. Macduff surely has his old responsibilities to look after, as well as new ones, and the fact that his actions upon killing Macbeth go immediately toward recognizing Malcolm as the new king suggest that he is wholly devoted to the traditional rule of law, and is untainted by the corruption and egotism that plagued Macbeth. Macduff might serve as a sort of bloodhound, ferreting out the kind of supernaturally-inspired treachery that led to the events of the play.
An element missing from the play is, indeed, a sense of what comes next. We might look to Beowulf for inspiration here, for in the final lines of the story Wiglaf recounts several of the blood feuds that will surely be rekindled in the wake of Beowulf's death. Likewise, Macduff might look to securing Scotland's borders from invaders, inside and out, who might seek to exploit the weakness of Malcolm's regime as it establishes itself; perhaps a return of the Irish or Norwegians from Act 1, or the families of the traitorous nobles that aided them. Macduff might also take pause to consider where Macbeth acquired the prophecy that failed to save him, leading him on a literal witch-hunt.