When it comes to tier progression for zombies, the gauntlet mode would be the best option for adding tier progression support due to these factors:
An issue with tier progression in classic matches would be due to how easy it is to just set up a game with 3 bots on casual difficult on Blood of the Dead or Classified and just go AFK while leaving the console or PC running all night long in order to collect an extra 10-13 tiers each night or even take it all the way and just have the game running 24/7 with short breaks to collect the tiers or to apply console or Windows updates as well as any game patches in order to rack up 29-30 tiers a day if it is timed based just like multiplayer and blackout.