Warzone will remain standalone and on the same engine as it is now but Black Ops Cold War content will be imported into it sometime in December. You can only create one class from content in either game so you can't have a gun from MW, perk from CW, lethal form MW and then a non-lethal from CW.
I'm gonna be interested to see if they will improve the weapon animations of guns going into Warzone from Cold War becuase the MW ones look great but the CW animations look severely downgraded.