From a different angle why couldn't you have multiple elite trait lines? You would just have to have the traits be 1 major and 2 minor. The major trait would follow the changes (for example if you have reaper major but scourge minor you would have reaper shroud and access to the scourge traits, weapons and skills but not mechanic (shades)). It would need a few skills fiddled with but I think that would greatly allow build diversity and fun combinations. This way you don't have an extra 4th trait power creep bar full of modifiers but still a lot more flexibility. Plus any elites in the Future could add to the pool of choices.