What about starting with the flu and giving it an HIV-like ability to wreck your immune system? Is the “attack” part too intertwined with everything else to be able to do that sort of mix-and-match operation?
the flu gaining deadlier characteristics via engineering is more realistic. unfortunately, i believe that is well within the scope of our present capability. the exact magnitude of how dangerous such engineering could make a virus based on the flu is unclear to me, but i'd estimate somewhere between "globally apocalyptic" and "continentally destabilizing".
the mixing and matching of attack characteristics is probably possible under certain circumstances, but i don't know of any specific instances where it has been done. theoretically, it's easy to swap A for B, but making such changes nearly always has unintended downstream problems.
in the lab we used to do all sorts of mixing and matching, but for defensive characteristics (mostly to see if certain isomorphs were more vulnerable than others).
long story short, generating virus and isolating it is a real PITA for a slew of reasons. experimental cycles might be as long as a week for each trial of "mixing and matching".
