Viruses that could cause severe, catastrophic pandemics?

So in the past decade we have had a couple of pandemics that have not proved to be as bad as first feared, SARS and Swine flu. In both these cases I believe that we have been incredibly lucky that they were not as severe as first feared and if either of these viruses mutated into something much worse I believe that the world we see today would be very different. My question is that what viruses, such as bird flu could cause a pandemic with catastrophic severerity in the next decade? What would be the most likely result of a pandemic from each? Fatality rate?