So, I do have a soft spot for the waves of chaos burning all in their wake but after all these years chaos doomstacks have gotten a bit old hat.
I think it would be nice if CA did something to vary up the endgame a bit. Off the top of my head I can think of two potential solutions.
Just randomize the end game threat, be it pure beastmen spawning in forests around the world, Ork Hordes coming to crump all da gitz, tides of the dead emerging from historic battle sites or the Dreaded vermin tide.
This would keep the player on their toes never knowing what endgame threat will emerge to challenge them or where from.