What COULD Gaz add that would keep you grinding away endlessly?
With the new personal difficulty sliders will power creep really matter that much?
If not...
What about new comms that upgrade medals and insigs to 69. But have it be a much much more meaniful upgrade than from 63 to 66.
These comms could drop in Danger Room, Ultron, and X-Defense (replacing the old useless currency). It would take a TON of comms to get the upgrade. Maybe they could also be used to enchant your Legendary and again cost a ton. Completely lvl 60 story mode could also reward them.
It would certainly give me a reason to play those modes again.