Hi Rachael,
I also agree with you that the flu might not be the next big one. You hit the note on point with preventative vaccine, medicaitons and the measures to decrease the rate of transmission. However, I believe that the next big one might not be an old virus that builds up immunity against the drugs available like XDR TB. I think so because the next big one could very much be a new virus that still has not had the chance to spillover or has already spilled over and doesn't have a cure coupled with high transmission rate.